Can you actually measure him for hat size per some of the fancier helmet brands? That should at least give you a size range to look at and allow you to rule some out.
You may be surprised, though. I generally think of myself as having a large head (cannot wear most one size fits all stuff at all, gives me a headache or pops off) and my current helmet is actually a M Ovation. (I would have needed a large if I’d wanted to do hunter hair, so right on the top edge of M, but still. I do not normally consider myself as having a M sized head. )
I would also recommend, if you can’t get to a tack store to try on, seeing if you can order more than one size/model and try on at the same time. Ime that makes it much easier to compare fit than if you get one, try it, send it back, etc.