My younger one - when he was preschool age - was "homeschooled" too for a while. He had a book and lessons and activities. He LOVED it. He went with us to the hs groups/classes - and of course - as long as he was quiet he could sit in, too. If he got antsy, we went out and had different "lessons" - or play as the case may be.

To this day - he's an 8th grader now - he bitches about the fact that he wishes he was still hs'ed! I sent him to regular school for the Spanish immersion - a choice I don't regret but wish that traditional PS was a little more child-friendly. If it weren't for the Spanish, he'd be hs'ed in a nanosecond.

At any rate, you say "engage him in THE activity" - he probably needs his OWN activity - or at least a more preschool friendly version of the activity. It's not easy, but it is doable.

The fact that you have 3 uninterrupted hours is huge. Besides, how many more hours do you really need? (Count up the actual "instruction hours" that take place in a PS. Factor in how much more efficient hsing is.) Schedule your older son's work such that he is doing the majority of his independent lessons/work when the younger one is around.