I had a horse with arthritis in his neck. We did injections yearly, but as he got older we were doing them every 5-to 6 months, but he probably needed them every 3 months. Arthritis is difficult to deal with an it can deteriorate and get worse fast. I would consult your vet and put a plan together on how to keep your horse sound and most importantly comfortable. After all, arthritis can be painful.
Keep him moving and see if injections or joint supplements would benefit him
I would say walk a lot, if he has arthritis, don't work too much with him, don't forget that he has now limits more than before! You should ask a vet to get some advices, it depends on how much he has arthritis. You should play with him and see how and when he's fed up with working :)