Are you sure that the remaining swan is not the mum? The reason we ask is that it is not normal behavior for the remaining cobs to allow a cygnet to stay in the area unless they are all juveniles. The parents should fly together.

If it is not the mum, then eventually the mum will either return or the young swan will take off to join the others.

If the cygnet does not leave, then you might want to contact the RSPCA or Swan Support to capture and examine the bird to ensure that there is nothing wrong such as injury or illness that renders it unable to fly. The Regal Swan