My daughter took her first Oklahoma white tail doe at nine years old with a cut down Stevens 200 in .223 Rem with 55 grain Vortx ammunition. She used it again for her second season taking another doe. As long as the range was reasonable the .223 worked very well. She switched to a .300 Savage at 11 years old and never looked back, the .223 is just a varmint rifle now.
Let your son decide what he wants to take, I bought a custom .250 Savage and downloaded .243 ammo for her to try. She liked the .223 and shot it much better than the other two rifles. I saw no reason to force her to shoot something else.
Then we discussed shot placement from different angles, and used deer anatomy targets for her to shoot at. She didn't pull a trigger on deer that were alert to us, and we talked through the shot. I tried to stick with broadside shots only, but I talked her through breaking a shoulder on a quartering to shot on a doe. That 55 grain TSX bullet was more than up for that task.
I'm sure whatever you use your son will be successful.
