There is a possibility that your pair of swans are both females which would mean they would go about the usual nesting/egg laying process, but obviously the eggs are not fertile. If you truly have a male swan, then one or both of the swans are infertile. Swans and other wildlife will not continue sitting on eggs that have no chance of viability.

Finally, the swans may be of an older age. Swans as well as other birds come with an innate number of eggs to be laid throughout their lifetime. If the swans began laying eggs at a very early age, then they may lose the ability to lay eggs earlier in their life as they age. Conversely, if the swans are older as they begin laying eggs, then their egg laying capabilities will last until a longer period in their life. The Regal Swan