If he’s 9 you definitely want a low recoiling round, otherwise he may never want to go again or learn to flinch.

I suggest a 243, 257 Roberts, 250 Savage or factory low recoil rounds in a 25-06 or 270.

AR-15 platform Grendel will work, 6.8 SPC too

Good luck and happy hunting

Drink and be merry for soldiers die for your freedoms.

Semper Fidelis!