You might want to have the horse checked out by an equine chiropractor. He could be out of alignment somewhere that makes it uncomfortable to move normally. A few vets have also had training as equine chiropractors, but most have not, and the concept of horses having a misalignment has only become known in the last 20 years, so many people do not realize that horses can & do sometimes need to have their alignment checked and corrected. If you have ever had an un-diagnosed misalignment, you would probably be aware of how painful a misalignment can be. But since this is, within the horse world, still a fairly new concept, mis-alignments are often missed.