There is no standard ring size for a man. The size is different with every man. The best thing to do is to get a ring that he previously owned and use it for fitting. Alternatively, get a ring that is big enough and then have it adjusted later.
He's what my mom used to call ' a tall drink of water.' If he wears a ring now find a way to get it off him and trace it on a piece of paper. If that's not possible then buy the ring with the understanding it can be returned by the store or exchanged for the right size. I'd go with a size 8