Yup, check out smartpakequine.com
They have quality supplements that actually work.
Anywho, MSM is what you want to get him on. It's super cheap, and extremely effective for most horses. Keeps the swelling down which helps slow the progression of arthritis and also does wonders for an injury. (It takes a week to be effective, though). Get 10,000mg per dose, though. A lot of manufacturers seem to think 1,000mg is perfectly fine, which is stupid, since that's the human dose.
Other than that, pick whatever else you want. If you have money, go with Cosequin. It's a good supplement that works very well, but it's a bit expensive.