I tried just about everything, and I started my son too young, but if I had it to do over again I would start with what he has now.
I bought an older Uggie SxS and bobbed both ends. The guns are cheap so I didn't feel bad butchering it and they are as simple and reliable as a hammer. Open chokes don't hurt for a kid just learning, so I cut the barrels to 25" and left them cylinder, I may jug choke them as he gets older. My plan is to add progressively longer recoil pads as my son grows. There are plenty of older SxS and O/U guns that would fit the bill.
I would suggest buying a 12 gauge gun, and using subgauge inserts to fire 28 or 20 gauge shells until they are no longer needed. The weight of a 12 gauge gun will help negate recoil. As long as the barrels were cut and the gun still had decent balance a little extra weight was OK, at least for my son.
It is much easier to see that a break open gun is open and safe than a pump or auto. CJ started bird hunting with me this year and he carries the gun open until the dog goes on point. I don't feel as comfortable about the pump, and I would be even more worried about an automatic. The self resetting safety on the break open gun is also a comfort.
Good luck to you and your boy.