If you can watch him, see if he plants the foot and walks while on land. Sometimes, swans will hold a foot to rest on their back or dissipate heat. The foot can be opened or closed. If the swan closes his foot, it will look smaller.

There is always the possibility that he stepped on something or injured the foot/leg and cannot place weight. If the swan does not come on land or he does not place the foot down once he is on land, you would need to call the animal welfare group and have him checked out. Thanks for caring for this magnificent bird. The Regal Swan