If you want to go 243 get a 6mm creedmoor. 95gr lrx @ 3200 fps.Because of my coastal locality, unless I want to commit to reloading-only, which I don't, I have to go with popular cartridges. So for this "weight-class" that means either .243 Win or 6.5 CM. And between those two... the .243 Win would appear to be much more mild in recoil. I don't see the boy having any problem with .243 Win. And hey... I can always go back and buy a 6.5 CM in the future if I feel it's needed later.
EDIT: Meanwhile I've already got .30-30 Win and .308 Win. So can always elect to use them if I feel the need to. But I feel confident an 80gr TSX will handle what I need it to.
99.9% of ALL HUNTERS have no business shooting game at 650 yards. Just my opinion.99.9999999999999999% of 13 year old kids have no business shooting game at 650 yards.