As SAcres said, circles are a bad idea.
My horse has an old injury to his stifle, so a bit different then a locking stifle. But my vet told me, no circles, no trotting and very light cantering. Hill work is good, backing is alright, but a lot of times it hurts for him to move backwards, so don’t do too much of it. Also putting poles on the ground or making him step over small logs. No collection whatsoever either. But again, an injury to a stifle is a lot different and has different treatments and lameness issues than just a locking stifle.
you can try a homeopathic remedy to help with his pain. The only downfall of a locking stifle is no matter what you do, it will always lock up. Maybe only once a year, maybe more as he ages..but it’s not something that will go away.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."