From your description, it sounds like "spraddle or splayed legs", which due to an injury to the legs, causes the legs to go out toward the side. You need to get him immediately to a vet to have any chance of correction if this is the problem. The vet will need to place a "hobble" on the legs to get them to go back in place. The longer this is allowed to go untreated, the less successful that the treatment will work.

If this is not "spraddle legs", it could be some type of neurological issue caused by disease or poor nutrition.

In any case, a licensed veterinarian with experience in waterfowl or birds is needed to diagnose and treat whatever is causing the problem. The Regal Swan