I know when i was his age i was introduced to the .22. I was a smaller kid at that age as well and i enjoyed shooting it. It hardly kicks the shot isnt very loud but i would suggest you taking a few shots first to show him what its like and so he can get used to the noise. After that he should be pretty comfortable around guns so i would upgrade him to a smaller calibre for deer such as the 22-250, .223, or even the .243 i think would be good enough for him but then again you have to take into consideration of his size, smaller boy smaller gun, bigger boy im sure he would beable to handle a bigger gun.