Take him to a GOOD shop that handles several makes and models and find one that fits him. Then let him work on saving enough to get a new or used version. The A400 is an excellent shotgun and I'm guessing it fits him well. Going to an O/U, the fit will be equally if not more, important.
Not knowing how old your son is or his size, any 'growing up' he has left to do will have an effect on the fit as time goes on.
Some brands to look for:
Browning (Citori and the Cynergy)
F.A.I.R (one of the Guerini family)
Or, just to keep you and your son busy, here's a web listing a BUNCH of manufacturers. aka = shotgun porn: http://shotgunning.tripod.com/