The best thing for arthitic horses is daily light exercise. You can't "weekend warrior" these horses, you have to ride them consistently every day. The best is doing a little bit every day and slowly working up to maybe a half hour or hour of light riding per day, mostly walk. See where his limit is and stay below it.
As far as joint supplements, any glucosamines fed orally will not be absorbed. MSM has been shown to be absorbed but in this case I dont think it would have much effect. You might as well top dress his feed with powdered sugar. The most important things are lots of turnout with encouragement to move around (buddies, placement of hay and water in opposite corners, etc..), maintenance of weight (body scores of no higher then 5 or maybe 6) and only after that more therapy.
I like asprin, daily for an anti inflammatory as it is easier on their stomachs. You can get industrial sized bins of it, grind it up and make a paste like bute or top dress the feed if he'll eat it. I also like cold hosing over liniment for topical treatments. You might also look into IM injections of Adequan as in some cases it has actually been shown to regenerate cartiledge. I'e had great success with it. However it is expensive. You might also talk to your vet about treatment with Tildren, IRAP therapy, etc.. but these are also expensive options. The lowest cost alternative is probably Legend IV and an IM glucosamine once or twice a month. This will replace and actually work better than oral supplements and be of a similar cost.
They say money doesn't buy happiness -- well happiness doesn't buy horses!