How big is he? How old? Any idea? A vet should be able to tell. Certainly Border Collie mixed with something - I'd guess Saint if he's big, he certainly has more of a Saint/BC coat than a Boxer mis would, Boxer's coats are sooooo smooth and tight! I know you're seeing Boxer in the set of his jaw, but the undershot jaw could be lotsa things, or just a quirk of his own.

What a handsome, handsome boy!