30-06!

With all the open ended variable and uncertainties, definitely the 30-06 and he'll be set for life to go most places and do most things from prairie dogs to moose and elk.

Ammo is cheap and plentiful, plus the variety of bullet weights and power loadings is outstanding:
** "Managed recoil" for plinking
** "Regular" for deer
** Federal "High Energy" for more serious game.

Very versatile and very much NON-specialized, that is its strength!