Bigrob, my nephew (9 at the time) killed his first with my 12 shooting a remington express long range 6 shot, saying that I wouldn't trust a shot over 20 yards. Luckily his was only at 11 steps !!
They are now 11 and shot their first "turkey" load over the weekend, they took it quite well and never flinched. Before I would not let them shoot (practicing) a turkey load, I always stuck a heavy dove load because like you I didn't want them to get "gun shy". They both have killed 2 turkeys each and have never made one comment about the gun kicking after shooting their turkeys. I think like the rest of us, they get caught up in the moment and never realize the kick from the gun.
Good luck to you and your son and hopefully he will be your next lifelong turkey hunting partner !!