I have been told, by People Who Know A Lot More Than Me, that way way back in the old days, tuning may have been a few Mhz lower than it is now, for whatever reason. I am under the impression that people who recreate music of the Renaissance & such, routinely tune lower. Indeed, even the great folks at the Boulder Early Music Shop sell an A415 tuning fork as well as an A440 one.

If he wants to play low, fine. But if he's going to play with everyone else, then he's going to have to get over it.