He's 11. Long range work requires practice. A 243 or 6 Creed would be my choice, and I go with a Tikka T3x or, better yet, a Barret Fieldcraft.
I hunt deer and antelope every year and guide for antelope. I have seen a LOT of hunters succeed and fail. A 6mm is fantastic for deer and antelope.
I can hunt them with any cartridge I want and use a 105 bthp from a 243. I may switch to the 6 Creed when my 243 barrels are gone.
Practice, bullet choice, optics and optics mount are just as important.