It really depends on how bad the confo fault is. There are lots of sound knock kneed horses that are worked hard. What worries me is that if he had to have corrective surgery and now requires corrective shoeing to stay sound, then the problem was very bad to start with. I would definitely check with a trusted vet and just see what they think. My best guess is that if he is sound enough to ride at all, it should probably be just easy riding like on trails or for pleasure. Pictures would help but if you were to put any undue stress on those joints by asking a lot from him, he may go completely lame. He would also be at risk for early onset arthritis because of the uneven wear on his joints. Pictures of his front legs from the front and side would definitely help with suggestions here.