i started shooting a 308 at 10 or 11. i didnt have a problem with it...but i grew up shooting so i knew HOW to shoot. i think knowing HOW to shoulder a gun and take its recoil plays a bigggg factor into the punch you take...

a good aftermarket recoil pad on a 308 and id say your son wouldnt mind it...so long as he has it tucked in his shoulder properly...train him on a 22 or pellet gun if you havent already...

or you could try the manage recoil ammo...i dont know much about it other than they are supposed to recoil less..

good luck with your son