Have you had an avian veterinarian exam him? The University Tennessee at Knoxville has a veterinary college, so you might check with them to find an avian/waterfowl vet in your area.

Sometimes, swans will stand on one leg if there is a break, soft tissue injury, pain, fishing tackle, etc.

If he is in pain, this can be mitigated through meds such as Medicam, etc. You can help him by keeping him in a safe secure environment (penned) and provide a zero entrance water feature so he can butt scoot in and out of the water. As long as he is not in pain, not infected, no open break, he could do just fine as long as he can access water without having to do any climbing. Trying to fix the break may not be advisable at this age as he may have problems assimilating the anesthesia. You just need to check with a vet and see if there is a break, how bad and if he is in pain. You certainly do not want him to him to be in pain or suffer. The Regal Swan