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This very well could be a physical problem and likely is since it just started happening and is happening several times a day.
I've never heard of excitement pee in an adult dog. This is fairly common with puppies because they aren't yet old enough to be able to always control their bladder and particularly when they are overly excited. Adult dogs have extraordinary bladder control - FAR more than us bladder-that-must-be-the-size-of-a-pea humans. It isn't normal for an adult dog to have excitement bladder control issues, and it just isn't something that I can see would happen overnight several times a day without some underlying physical problem.
In the meantime, have the dog empty their bladder completely before they are in a situation in the house with someone that causes them excitement and thus peeing. Have anyone that meets/greets the dog who makes him excited do it outdoors (wearing a raincoat! haha!) until the dog has calmed down.
Don't have your son or anyone else that seems to trigger the excitement peeing try to physically calm the dog. IGNORING the dog is what will make him calm down though at first this might take awhile. Paying MORE attention to the dog will just rev up the excitement meter. Attention for a dog is a REWARD for desirable behavior, so you never want to give attention to a dog that is NOT in a calm state. Giving the dog attention ONLY when they are in a calm state teaches the dog that if they are calm they will get the attention they find desirable and that when they behave like a nut they don't get the attention they find desirable. Very quickly the dog figure this out and knock off excitable behavior. Like anything else with teaching a dog what behavior is wanted at any particular time consistency is absolutely key, and that it takes many repetitions (about 30 or more) before the dog really learns what specific behavior is wanted when told or at specific times or instances.
And of course, all of this will backfire if the person wanting the dog to be calm is not feeling calm and relaxed themselves. Dogs react to what us humans interacting with them are feeling and can detect what we feel even when we don't realize it ourselves. It does no good to try to calm a dog when we aren't feeling calm ourselves. Instead of calming down the dog will react to our pent of feelings and keep acting out which of course creates a cycle of higher frustration (feelings) on our own part and thus higher excitement/misbehavior on the dog's part. And that's the kind of thing that makes us think "Oh for crying out LOUD, what the hell is wrong with this %&*@^ dog!" It's not the dog messing up, it's us though we tend to get stuck trying to figure that all out and what in the world to do about it.
If the dog is particularly reacting to the presence of your son, try this: instead of your son going and giving the dog attention especially when the dog is in an excited state, have the dog outside and your son somewhere away from any physical interacting with the dog while the dog can see him and know he's there. Have your son completely ignore the dog whatever the dog is doing while concentrating on being calm and relaxed himself. Have a chat with him or snack or whatever to distract his own interest in the dog and what it's doing. Eventually the dog will give up the excitement and calm down AS LONG AS you and your son and anyone else around is feeling calm and relaxed though it may take a long time because as of now the dog has learned that when he acts like an excited nut he gets the attention that he wants, and the purpose is to reverse that.
Only when the dog is calm and your son is calm have your son go into the yard or where ever the dog is but continue to totally ignore the dog. The dog can watch him, sniff him, walk around him in circles, etc., but only when the dog is calm again by your son's physical presence can your son then give the dog attention and ONLY calm and relaxed attention. The SECOND the dog starts to show excitement again, your son has to immediately stop giving the dog attention and go back to ignoring the dog. This will start the dog figuring out that he only gets the attention that he wants from your son when he behaves in a calm manner which is the opposite of what has been going on before and what he's learned.
However long it takes is how long it takes. The first few times may take a LONG time to get to the point where your son can give the dog calm attention while the dog remains calm himself. Don't give up or give in. EVERY time the dog begins to show excitement with your son, your son has to calmly and immediately withdraw all attention... don't look at the dog, don't touch the dog, don't talk to the dog - completely ignore the dog until he is calm again.
Dogs are VERY clever in figuring out behavior that gets them what they want even when that behavior is not what we want but we don't know how to change it or even how it came about in the first place. So far the dog has learned that he gets the attention he wants from your son when he behaves overly excited to the point that he's peeing on himself and anything else in the vicinity of his tinkle stream (though the sudden peeing is likely an underlying physical issue). Unfortunately, us humans don't always know what it is we're doing wrong that is causing our dogs to act in what we feel are inexplicable ways. Very often it's a hell of a lot easier for other people to see what the problem is than we can ourselves because we're the ones frustrated, confused, pissed off, or whatever that here's this problem with the dog and we don't know why or what to do about it, and dammit, the dog never did this crap before and AAAAARRRRGGGGHHHH!!! 
We've probably all been there and done that many a time. LOL!
If this is a male dog, has he been neutered? If he hasn't been neutered this might also be a marking issue though I really doubt it. Unneutered males can often just piss all over the place constantly and particularly if there is a female in heat in the area that they are able to detect. If your dog is an intact male is it possible that your son in particular is carrying the delectable odor of a female in heat on his clothing or skin? However, even an adult unneutered male that is marking shouldn't be peeing on themselves and not peeing in copious amounts. Normally when intact males mark they lift their leg and let loose just a little bit of pee but they do it all the time on damn near everything inside and outside though plenty of intact males never mark at all at least not indoors. I also think the level of marking has a lot to do with the vicinity of intact females especially one in heat though you may not know that one is around that the dog can detect... sometimes it seems that intact males can detect a female in heat half a world away! I REALLY don't think this is marking behavior though, but I thought I'd throw it out there anyway.
Could you describe the level of excitement the dog experiences when he has this peeing issue? Is he moderately excited or absolutely flying out of his skin excited? Does he only have this excitement peeing problem with your son or is it your son in particular that causes the most excitement and thus the most peeing? Did the peeing suddenly start with the appearance of your son in particular? Is the dog much more exited by your son's presence than anyone else's? Does he do the excitement peeing with other people that aren't your son or does he only do it when a guest is around that causes him to be excited?
These are all things you'll want to tell your vet to help try and diagnose the problem whether it's a physical problem, emotional problem or a combination of both. It's always best to give the vet as much detailed information as possible - you never know when a little tidbit of information might be the lightbulb that goes off for the vet to pinpoint what is likely wrong.
First things first though, call the vet and explain the problem as detailed as possible. Sometimes just a phone call will have the vet knowing what the problem likely is. Still, take the dog to the vet for an exam anyway since there very well likely is some underlying physical problem where any excitement at all causes the dog to not be able to hold back his urine. Since this peeing problem has happened very suddenly and several times a day I'm really thinking there is some kind of physical problem going on where any excitement at all causes the dog to not be able to physically hold his urine... maybe a urinary tract infection or the beginnings of one.
Best of luck, and I hope you can get this problem resolved. Flying pee is just no fun at all.
