Personally I would try to find a used Beretta 303 or 390 in a 12ga. cut the stock down for him. As he grows stocks are easy to find and relativly cheap. The automatic will cut down the recoil alot and the 303 or 390 cycle 1oz shells well. Also the guns are pretty light and can be had reasonable. Having a 12ga. he can shoot turkey, quail, dove, and other upland birds without ever being under gunned. Not that a 20ga. won't do the job I just think the 12 will be more versital.

Most of your 12ga automatics don't recoil as badley as a fixed breach 20. Also the Berettas are very reliable and don't have to be cleaned as often as the Rem 1100's or 11-87's.

Good Luck

Alan