have you though about a 12 ga. the 20 ga with 3" loads is equal to the 12 ga. 2 3/4 as far as the kick goes.then the 12 will grow with him for years to come.
get a 12 ga. 3" and he can shoot 2 3/4 shell till he grows to it.
Just a though.that what my dad did for me some 40 year ago.

luck on which ever you get.