The parents may know something is wrong with the cygnet. If the cygnet has an illness, the parents do not want it to infect the rest of the family. If the cygnet is weak or injured and cannot keep up with the rest of the family, the parents do not want it to lead predators to the family.

The best thing would be to get him to a wildlife rehabilitation center, a waterfowl center or somewhere that the young swan can be raised around other swans so that if he does not have anything wrong with him, can grow up to be released onto a pond in a captive setting. If you release him back to the family, the parents will kill him or he will be killed by a predator. The Regal Swan