I think this a great idea Moosejay! here are a couple ideas though since you don't seem to mind spending a little money on this kid.
Offer him a bonus at the end of the 4 months if he does infact complete the whole thing and reach his goal. You said he would get 1k a month at the end of the month, maybe add in another 1k at the end of the whole 4month deal if he sees the whole thing through...just a thought.
Also, what about when the 4month period is over? It would be nice if he continued to train and eat right. maybe he'll be one of the lucky ones and get hooked all by himself (in a healthy way) but most don't. Of course I understand that even with some sort of financial incentive he may not "get hooked" but it couldn't hurt. BTW, this is where the trolls come in and say "only an idiot would want to get hooked on training" lol!
Supplements that are worth while IMO:
A good isolate powder
Fish oil
Corral calcium
A good fiber supplement
...and Juice Plus+
other then that, there really isn't a need for anything else, especially at his age.
BTW, I'd like to hear how all this pans out by the end of the summer.