A 20 gauge should work perfectly fine. Pattern your load, know your range and there shouldn't be any issue. But, I will say this. My first shotgun was a 12 gauge. I grew up shooting a .22 and got my first 12 gauge at ten years old. My dad taught me how to hold, shoot and follow through properly, and recoil was never an issue. NOW, when shooting clay pigeons I was done after a box of shells. I couldn't shoot the thing all day. But for some practice shots, and for a hunting situation, especially a turkey hunt where one shot is usually what it takes, I wouldn't be afraid to try him out on a 12 gauge. Have him shoot yours and see how it goes.
-Jake