I’d like one rifle to be all he needs even if elk at 400 yards is a likely shot.
Well, you've just described a 270 AND a 308, 7/08, 280 and 30-06 (and anything that fits in between those). I think starting a new hunter, irrespective of size or age, with a magnum is just setting them up for failure/problems. Choosing to shoot a lighter recoiling rifle does not make one less of a man/woman. Arguably, it displays one may even be smart. Good luck in your quest and kudos to you for helping another prospective hunter.