As a little kid I killed a couple birds shooting 2.75" #6 heavy game loads. If it were me, I'd let him practice with some low brass bird shot so he won't be afraid of the gun and on the hunt put a regular turkey load in there. He won't ever feel it when he shoots a turkey.
Also, I think Federal is loading some 2.75" Heavyweight loads this year. They shouldn't kick too hard and would extend his range way past 20 yards.