What size shirt would a guy wear if he is 6' 5" and 315 pounds?

I'm trying to buy a shirt for my bf, but I don't want to ask him outright what size he wears. About what size would a guy whose 6 feet 5 inches tall and about 315 pounds wear? Hes not fat, just chubby, but built with very broad shoulders and has a larger overall bone structure.