270 win, 30-06, 7-08, or 308 win would be my recommendations.
This. Ammo will cost far more than the rifle over the long haul. I'd go 30-06 and not look back. For glass, get a used fixed 4x or a Burris FFII 3-9 with a ballistic plex.
But there is one MASSIVE problem with this setup: he'll never be able to justify another rifle without becoming a complete loony. Which, with you steering the boat, is a real possibility.
Okie John