What to get the kid brother
I have to go along with the .30'06, for all the reasons above stated, plus one area that hasn't been mentioned: When (not if) he gets into handloading, he'll have the most versatile cartridge ever designed, and the widest variety of bullets, powders, and loads, to work with.
As to practice, practice, practice, I couldn't agree more. But what you're practicing mostly is sight picture and trigger squeeze. This can be done with a good-sighted .22 LR, which I would suggest getting. Then the cheaper-ammo .308 argument goes right out the window! Get him a couple bricks of quality .22's, and tell him to come back for a couple more when those are gone.
The kids (above) who bragged about shooting Dad's 12 ga. ... What did they shoot, shoot, shoot all summer afternoonÑNot a big noisy12, certainly not Dad's deer rifle, nothing to brag about, just honing skills with the li'l ol' .22. Then it was potting rabbits in the fallÑand Mom doesn't like picking shot out of the bunnies she frys.
Then before deer season Dad takes the kids out to the range and each kid gets a box of '06 to burn up. The kids always complained about the kick at the range (but not enough complaining so Dad could say "well, if you'd rather not go deer hunting...") On a deer, they never even noticed. But having burnt up all those .22 rounds, and studied bunny and deer anatomy, they knew how and where to place the shot, and THAT, my friends, is the most important thing.