There could be any number of issues as to why the young swan did not fly with the parents.

There could be something wrong with the bird and he cannot fly, either birth defect, illness or his wings are just not fully developed.

He chose not fly and the parents will return in the next couple of days to join him.

We would suggest you wait a couple of days as long as there is no visible illness or injury to him and as long as he is in a safe place so there are no predators (domestic or wild such as dogs, cats, bobcats, humans, etc.) that he cannot escape if he cannot fly.

If the parents do not return, you might contact a local wildlife rehabilitation facility and see if they might try to capture the young bird and get it to a veterinarian to determine if there is something wrong. If nothing is wrong, they may have to keep it in a safe shelter for the winter to make sure that it can survive inclement weather or find it a permanent good home such as a farm.

Please try to find a no-kill shelter for the bird as many states are directing wildlife facilities and veterinarians to kill Mute Swans or refuse to treat them. The Regal Swan