The little 55gr Hornady SP will do everything that needs done. Also any Barnes bullet. And don't forget the Sierra 63 and 65gr.
As for his age, my nephew started with a 22lr at about 4. Got his first deer rifle, a Model 7 CDL in 260Rem, for his 6th birthday. He killed his first buck with it that fall using my light handloads. Kids have different interests and mature at different levels. As his father, you know better than anyone whether he's ready to deer hunt or not.
