Yup, he needs to be on at least MSM. MSM is an anti-inflammatory and is extremely effective for most horses. Arthritis progresses because of inflammation. If you remove the inflammation, you slow the progression of arthritis. Make sure you get the 10,000mg dose. Smartpakequine.com has a $6 bag for 45days.
If you have money, put the horse on a joint supplement. Cosequin is the best one out there, though Finishline supplements are good too. (You'll need to add extra MSM to most of their supplements though).
HA supplements are very popular. Feed-thru (Hylan, etc), HA supplements seem to work well. Ask your vet about Adequan. It's injectable HA (hyaluronic acid) once a month. (Loading dose is every week for 7 weeks). On some horses it works extremely well. On others, not so much. (My mare wasn't effected by it. Both of my friend's horses benefitted greatly by it).
You need to make sure you give at least 10 minutes of long walking as a warmup. Also make sure to do at least ten minutes of long walking (head down) to cool down. If you don't his joints could swell. That doesn't really bother a normal horse, but that swelling can irritate the joints then it contributes to more arthritis.
Ask your vet about Previcox. It's like bute, only without the side effects, and can be given daily. The 227mg pills are cut into 4 pieces, so 4 days worth. You get half a years worth of pills for $80. It's a painkiller and helps arthritic horses.