There is a possibility that something is wrong with him which is why the parents have run him away from the rest of the brood. Wildlife parents are not going to risk all for one baby.

Is the cygnet larger than the rest of the brood? If so, he may be able to get along as long as the parents are somewhat closeby. If you approach him, do the parents pay any attention? This will be a good indicator if he is still being protected.

If not, then you will need to determine if he can fend for himself. Are there predators in the area, i.e., bobcat, fox, wolf, coyote, etc. If yes is the answer, then there is a concern not only for him, but the other swans.

Is he pinioned (Cannot fly)? If so, then you may need to find him a home or contact someone with other swans to see if they can find an eventual mate for him and a good home. Otherwise, you might try to see if a wildlife rehabilitation center might take him. The Regal Swan