if you go with a remington 1100 or 870 I have some stocks that have been previously cut, they can be available if needed. no mounting hardware.
also there is only about 1lb diff between the 12 and 20 guage 870.
reminder also that if he needs a good amount of stock to be shorter you are restricted with the 1100/1187 do to the action spring/tube in the stock.
My vote is for the browning BPS for a pump gun..........the youth model actual has a better youth size stock. length of pull, grip to trigger ect ect.
For a Auto I would go with the beretta 3901 reduced length, this is the best out of the box auto loader for a youth in MHO
http://www.cheaperthandirt.com/71931-1.html My friend purchased one of these for his son and can hit a ace of spades and nock the spade out @ 100 yards with a scope regulary.
Just remember 20g vs 12g recoil is a moot point.............1 1/4oz slug out of either guage has the same recoil energy.
The lighter 20guage will transfer more of that energy to you than the 12g.
Check uot fourseasons in wobourn, they just sent me their email specials and they have 870 on special but is not a rifled slug barrel,,,,,i dont think.
A used 870 12g is ideal as it can be easily transformed into many configurations by a quick barrel change and or stock change.