As this is a long range site I'm assuming that there may be interest in eventually shooting further. While the 243 would get him started, that 80 gr. Bullet at 3400 drops below 1000 ft.lbs. Before 400yds.
The creedmoor set up I'm going to run can be loaded down to 243 recoil level now but loaded up later. It will push a 131gr. bullet over 2900 fps. That will maintain 1000 ft.lbs out past 800yds and be much better in the wind.
If it's going to be a cheap semi disposable rifle to be used until he grows out of it, go 243. If you are going to invest more, buy a gun in a cartridge he won't grow out of.
Just because it’s fun to debate... if he’s going to reload then go 6creed and send a 105 Berger with 1000lbs out to 800 with less recoil than the 6.5 with similar ballistics.... if he doesn’t reload the 6.5 creed would be second best