If you're really trying to help him fix his form, I would suggest keeping it really simple. You're on the right track with the putters, either a BB or wiz should work great. I would say on the mid ranges to just give him the comet. It will straighten out his form quickly because it doesn't hide any imperfections at all. If he's throwing 320ish as max D, I'd also suggest keeping the fairway drivers simple. Something along the lines of a few lighter DX teebirds and then a max champ teebird should be plenty. If he's working on his form and only throwing 320, then a new, max weight, champ teebird should be plenty overstable for his super overstable slot. (And in cases where it isn't, if he uses it anyway and learns how to shape that line with it, he will gain some extremely valuable disc skills).
So, to sum it up, I'd suggest something like:
2-3x BB aviars or wizards. Two for putting practice and during the round and one for driving
1 x Comet. Just stick with the one midrange for now. When he can make this fly straight, turnover, and hyzer, add something like a buzz or roc (if you go rock route, drop the comet and wear in rocs)
2x DX teebirds* in the 168ish range for control/distance duties
1 champ teebird max weight (star would also work just fine) for anything the DX birds can't handle
Feel free to insert leopard, stalker, cyclone, gazelle into this slot. They're slower and may actually work better for him. If you do so, then a banshee may be appropriate for the last slot.