taking lessons from a competent teacher.
Learning to play requires real work, and some people lose their enthusiasm when they realize this.
Playing original music in a band is an "all in" proposition. If he wants to make enough money to eat, I would suggest that he learns enough to be able to freelance as a sideman.
I've freelanced for over thirty years and that route, while economically easier, is still fraught with insecurity.
My advice would be, pick an instrument that is in more demand - bass or keyboards would be good. There are million guitar players and drummers out there.
Learn to read music and chord charts. This increases your gig opportunities.
Learn some basic music theory: the function of chord changes, what scales go with what chords, the relationship of keys...
Learn to sing some tunes - If you can pull off singing lead on some songs...great, but at least be able to pick out a part and sing backup while playing.
Learn as many songs, from memory, in as many styles as possible, jazz standards, Motown tunes, classic rock tunes. This makes you more hirable and helps you learn about what makes a good song if you have any aspirations to write original material. (Even the Beatles, Dylan, Hendrix, etc. learned other people's music)
Learn some basic business chops. Be on time, have a good attitude, have gear that is reliable and doesn't break down on the gig, dress appropriately for the situation...
Here's a good music business primer.
http://www.amazon.com/Need-Know-About-Music-Business/dp/0743293185/ref=cm_lmf_tit_4This is a big one. Realize that the odds of you being a rich star are incredibly long. Don't do this unless you absolutely love it and can't live any other way.
Good luck to you and your son.