You didn't mention the age of your son, but if he's young and you're concerned about recoil, you can go with a .308 win. There's plenty of inexpensive ammo available to use for practice since it's a common military caliber. I also assume that he'd be taking shots 300 yards or less and the .308 is very effective within this range.
FYI - I've downed many animals weighing close to 1000 lbs with the .308.
best of luck.
Ant
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