Re: What caliber for my son?
I went through this with my son who is now 15.
I started him on 22LR, then worked up to 223, 25-06, 270 and many others. He's small framed, but has learned to ignore the recoil from even a 300wm Rem Sendero with no brake although I wouldn't recommend him shooting it persistently so as to avoid developing a flinch.
Last year (14yo), he took a doe at 548yds with a 7RM Sendero with a KDF muzzle brake. He can shoot that rifle all day long with no discomfort.
To get him started on long range. I got a Savage 12F 6.5x284. It's a great rifle to learn on. The same rifle in 6BR Norma would be an excellent rifle for a young person to start learning to shoot long range. The weight/bulkiness of these rifles makes them not well suited to hunting. But, the weight and design tames the recoil without the need for a brake which isn't permitted anyway if you get into f-class shooting.
Many great suggestions were made by other posters.
If I could only pick one for both hunting and long range target and I wanted to be able to buy factory ammo in addition to handloads, then I'd lean towards 260 Rem. Savage chambers this in the Model 11 Long Range Hunter as well as the Model 12 Long Range Precision. I prefer the heavier barrel on the latter and would add an aftermarket brake to it.