I don't think a full power 270 load is going to help, or any full power load.
Both rounds are overkill. If you're buying a new rifle look at something smaller like a 6.5 Creedmore is popular theses days.
But if you reload, I'm a big fan of the versatility of the 06 over the last five decades. Load up some 150gr loads using the starting loads in the manual with 4895, or use the Rem reduced recoil loads if you are not a reloader, and that should be in the ballpark...they don't kick much.
Also stock fit and a soft recoil pad and use some shoulder padding at the bench.
Treat his flinch properly. Get the boy a nice shotgun shooting vest. I like my shooting vest with the padded leather shoulder pad, and even more after a long range day. They look good too and every skeet/trap shooter I know wears one for good reason.
Shooting from the bench is the worst position for a beginning shooter to start with a rifle that recoils and should be approached differently with adequate recoil protection.
If he has a flinch, that is very serious and he should spend some months behind a 22lr and work up to a 223.
Once established, a flinch is a neurological reflex problem that may not go away on it's own.
