Have you checked for wolf teeth? Could it be allergies- pollen , would a nose net help? Fixed a horse a used to ride.
put a piece of painter tape on his nose it distracts them so they wont bob their head
I think I would have the vet out to take a look, he may have something irritating him
Maybe it's something that has to do with his tack/bit or anything rider related? I'm not hating just wondering -- I have never seen you ride