Re: What calibre?
If it were mine (and I may be in a similar boat soon) I would go .280 AI for that game and distance.
If he is real serious about the 50 gr of powder limit though (which seems a hair arbitrary to me) I would vote 7-08 or .284 next. The 06 action calls out for an 06 cartridge, despite that a shorter one will work.
Empirical results rule!