if the boy is of a smaller size, maybe a single shot for the first gun, and then upgrade as he grows up?

I r=had a gun shop, and best advice I can tell you is have the BOY hold as many rifles as he can, they all do NOT fit the same , just cause you like something doesn't mean he will too
Biggest issue I have with YOUTH guns, is they never seem to actually make them youth friendly as they can!
the trigger to grip distance is almost ALWAYS the same as on a FULL sized rifle, and the point of a youth rifles , they have small hands, and fingers, NOT just arms and legs
why more MFG"s don't fit thee guns better I find crazy, there missing the boat so to speak when it shoudn't be that hard to correct!
some makers/model guns do make them better than others
so again, I suggest taking buy to a fe shops and have him hold as many as he can and go from there, any mid caliber shoudl be fine
.243-7mm08 are great one's too IMO and calibers most youth rifles all come in too