You can't go wrong with any of these calibers. Just look at recoil. Worst thing you can do to a kid is introduce too much recoil too early. As long as he doesn't flinch when he shoots it, shoot whatever caliber he shoots the best.
I have a CVA Scout 350 Legend that I would have started my son on. It doesn't recoil much more if any more than the .357 mag. No doubt the 35 calibers are my favorite.
