It sounds like you and your horse are very "in tune," so listen to your gut, and listen to any little clues he may give you, as well. The vet and chiropractor are excellent suggestions. Another possibility might be to have a horse massage therapist out to see him. Maybe you can find a combination chiro/massage professional.
And... hope this doesn't sound too kooky... you might try an animal communicator. Sometimes a talented communicator can help to shed some light on a difficult issue or situation.