I have had really good luck with the supplements from HorseTech.com--Horse Nutrition Supplements--Equine Nutrition on the Net
They are a good company to work with and they will custom blend any of their products or ingrediants, no extra/custom charges. The flax base really brings out the shine too
Another thing to consider is turn out time. Horses that stock up and/or have joint issues need as much turn out as possible. I know turnout/pasture is a hard thing to come by in San Diego (I grew up in the Oceanside/San Marcos area), but if you can swing daily turn out, it will really help him. Have him turned out with a friend, so they can run and play, for at least 3-4 hours a day. If you can't swing turn out, then you might see about having someone ride him at the walk/trot or hand walk him daily (no hot walks or lunging as small circles are hard on the hips/joints), or just turn him out in the arena and "free lunge" him for 30-45 minutes.