Sorry, I should have noted that Music Ace uses a piano as it's reference. If your son is willing to learn piano and you can provide an 88 key midi controller ($200 and up)this could still be a great choice. Otherwise try to find a similar program that is guitar based. I'm sure it's out there somewhere. You'll likely need to provide a midi guitar - but I still suspect this is the best way to go for a student who wants to learn to read without a teacher.
Marty
Marty