A good 20 gauge is a H&R youth model hold's a good pattern and is easy on the younger ones or those that are lighter framed.
That's what I started my son out on he was twelve and pretty light body frame,Now he's seventeen and bulked his muscle up he still likes to take it out and shoot it,sentiment i think.
I wish you luck in getting him adusted on a good gun he's not afraid of.
Also Good luck going after those bird's