You don'st say whether it is articular or non articualr ringbone. Joint supplements can be very useful for this. if you have a look on the riaflex-equine website you may find the free advice useful. www.riaflex-equine.co.uk
My horse has side bone and multiple joint arthritis and he is fed cortaflex, cider apple vinegar, Alpha bute (Boswellia).
Devil's claw is also widely used.
Things like Cortaflex are excellent for slowing the degenerative process down in joints and devil's claw and boswellia etc are very good as mild anti-inflammatories and pain killers. Cider apple vinegar is good for maintaining supplety as is cod liver oil.
What works for one horse, may not be so good with another, but it is definitely worth a try and adjusting things until you are happy with the results.
Regular gentle exercise is recomended for arthritis as well as year round turn out for this reason, if possible.
Magnotherapy is also recommended for arthritic/muscular conditions.