Hi, there are a number of ways to treat arthritis and relieve stifle strain.
Ask your vet (or find a really good vet if you don’t work with one regularly) if they have any suggestions in terms of your horse’s feet and if so, request that they work with your farrier-- for instance, many horses with stifle pain, and many older TBs are candidates for wedges on their hind feet and short “rolled” toes to help with breakover. All my TBs were kept in great shape despite arthritis, with careful shoeing which included short toes and sometimes wedged hind feet.
Sore No More gel/liniment before and after rides should become habit.
There is homeopathic stuff called Rhus Tox (poison ivy) and you can buy it in tasteless little tablets or in fluid form. it works best with arthritis that presents itself as stiffness/shortness in the beginning of a ride but improves with movement. ^^you can google this stuff and more homeopathic horse remedies. but ask a vet who is familiar with homeopathic remedies in order to use the one best suited to your horse.
also your vet might suggest a magnesium supplement.
also there is injectable glucosamine which isnt all that expensive, and you give it every 4 days to start, then once a week, and then once every two-three weeks for maintenance. injectable glucosamine has been proven to work better than it does in supplement form, and i can attest to its effectiveness as i currently use it on my big trakehner.
It sounds like your gentle rides involve a lot of walking, which is great, but i’m hesitant to encourage “light work only” unless your horse is unwilling or in pain or the vet has specifically advised you to only work him lightly. in my experience, the best way to keep arthritis at bay and to keep the horse from breaking down is to keep them in active regular work. i had an old (lived to 25, died of colic) y/o TB who had chronically stocked up hind legs, arthritis in his joints, and some navicular changes. we really focused on the ease of his ability to move well–we really stayed on top of his shoeing and we had him in “special” shoes to keep him able to move as comfortably as possible. he had always had stifle issues where they would catch, he’d trip, and they would hurt. it was actually much later in his life that we got the shoeing right and his stifles stopped being an issue.
soft ground riding is best, and 15-20 minutes of warming up at the walk and over walk poles.
good luck! hope this gives you a better picture of your options and your horse’s future!