DMSO might help with inflammation, however as already mentioned you really need to find the cause of the pain. Poor saddle fit, poor rider balance, unbalanced feet, lameness, arthritic changes in the spine....all of these and more can lead to back pain. When your vet determined that back pain was present, did he do a full lameness exam? If not and you are sure your saddle is not the problem, then that is the place to start. Pain in any limb can cause a horse to use itself oddly and lead to chronic back pain.
Licensed Veterinary Technician