Hi, good info on all the above posts. I won't go into safety and all that, you know and don't need to be told again.
Anyway, my two sons are both SxS shooters, they have been since the age of 7. At that age they both had 20ga SxS's that fit their physical limits. One of my sons is a bit taller and stronger than the other and his gun now is a 12ga SxS, the other still has a 20ga.
Each kid is different as you jolly well know. But kids today are bigger and stronger than when we were growing up. So they can handle guns with a bit more heft and can take recoil better.
As I said, match the gun to the kid. But, IMO, for Gods sake stay away from those damn Single hammer guns!!
It all depends on when he's ready to shoot both bbls, that is up to you.
Here's my suggestion for a good starter gun, make and model up to you.
25" or 26" bbl, 20ga and as far as ST or DT and grip style all up to you and him. Same goes for chokes.
Anyway, good luck and thanks for introducing another kid to the shooting sports and doubleguns in general.
Just have a great time and enjoy them as much as possible!!
All the best!
Greg