There may be something wrong with the cygnet. If so, the parents and the rest of the cygnets will shun him.

If you can pick him up and get him to a veterinarian before a predator gets him, this would be his best chance of survival. Even if the veterinarian can find out what is wrong with him, i.e., neurological or parasitic problem, he may not be able to be returned to the family so he will need to be raised in a safe wildlife rehabilitation setting until he can be released. Even then, unless he has been around other swans, he may become so imprinted to people that he may not be able to return to the wild. So, it is extremely important that a wildlife rehabilitator be consulted after a veterinarian looks at him.

There is no good answer in this situation and unfortunately, the only other option is to do nothing and let nature takes its course which will probably mean that the cygnet will eventually be killed by its parents or a predator. The Regal Swan