This is a tricky one, since scarf length depends on the individual's personal taste!! But I'd say "generally" that men's scarves are shorter than women's ones.
Something to check -- does he have any other scarves he uses that you could measure? How about one from a friend of his (another guy)?? Some guys like to have their scarf hang around their neck and others like to wrap theirs around a couple of times.
I know -- while the 2 of you are out somewhere together, just "happen" to stop in a store with scarves on display. Have him try one on and see how he wears it. Hopefully, this will give you some idea of the right length for him!!
Hope you figure it out soon!!!