Are you absolutely sure that it is the male on the nest? Typically, male swans (unless an individual or species attribute) do not incubate the eggs. So, if the swan is returning back to the nest, we would first try to determine exactly if the swan is the male or female.

Now, if it is the male and not the female, he may be staying by the nest hoping that the female will return or is using the nest for higher ground to rest if there are flooded areas in which the swan can not dry-out and rest out of the water.

If the female left, there are only two possibilities. She was either attacked by a predator and flushed from the nest or something scared her and she went to another area.

Usually, swans (especially females) will not sit on a nest where there are no viable eggs. The Regal Swan