Being at a yard where we have several elderly horses (one is 35 and not retired yet!) I have often dished out the feeds/meds to these guys.

They all (the oldies) have a liquid supplement called "No-bute" this is primarily made from devils claw and is a natural supplement to help relieve pain and stop stiff clicking joints (bute is a drug from the vets and has been associated with liver damage).

Also you may want to try the magnetic BOOTS on his legs (you can get a chain like thing or whole brushing boot length ones) we use the brushing boot type ones - I have read good things about the rug but if the artheritis is in his legs the boots may be better.