Your horse is looking very spry and handsome at 23! I have been told that Arabians reach their prime in their twenties. I hope he had a nice horse-birthday!
Years ago I read " Folk Medicine: A New England Almanac of Natural Health Care from a Noted Vermont Country Doctor" , By D.C. Jarvis, MD. The author talks much about the benefits of apple cider vinegar and honey to treat various human and animal ailments. I give my 3 y/o filly a splash of cider vinegar and about 1/3 of a cup of either peanut or corn oil in her feed daily. She has yet to complain...
She was so ill when we "rescued" her a couple of years ago that I figure that anything that might improve her overall health is worth a shot.
I know that capsaisin (sp?) the active constituent in hot peppers is said to be helpful for arthritis in particular. Apparently it can be purchased in a creme form. The extract ( in tincture or creme form) of the herb Arnica montana is also a good pain reliever for muscle strain, sprains and some have used it for arthritis, as well.
I have also heard wonderful things about flax seed and flax seed oil.
Maybe you might try an internal approach using the flax and vinegar and something external for pain?