If 400 yards is all you want, pick your favorite caliber that will shoot a 180 grain bullet at 3000 FPS. Mount a quality scope with ballistic compensated reticle (Leupold B&C or the like) and get after it. 400 yards is a chip shot with the technology in bullets and optics these days.
It does not get much easier than a 300 WM - H1000 - Win Brass - Win LR Primers - 208gr Amax. Talk about a cult classic. Then again the 210gr Berger is great too, and I hear (but have not personally tried) that JLKs are good. Like french vanilla ice cream and mom's apple pie!
I'm leaning toward getting him a rifle chambered for the 300 Weatherby cartridge and using a muzzle brake. A couple months back he made a one shot kill on an elk at 408 yards with a 25-06 that we handloaded with Barnes TSX. He would like a larger caliber for future hunts though, and recoil doesn't seem to bother him, but the brake would probably make practice easier on the shoulder.
Any thoughts on the ER Shaw Mark VII rifles. Maybe I should make a separate post on the rifle?
I don't have personal experience with brakes, maybe you already do. I'm told they are extremely loud to the shooter. That turned me off. Maybe some of you who advise against it would give your reasons, and or experience. I know I am courious if it is the extra noise you don't like.