Controlled movement is important for arthritis. I second the suggestion of long warm ups, a good joint supplement and a physio (ask your vet about the last two). What has the vet suggested so far?
The arthritis is in his shoulder
It all depends on where it is, your vet can have some options, you can try a joint supplement, massage and physio, lots of things. Make sure you do a longer, slower warm up when you ride to give the joints extra time to warm up and reach their full range of motion