The best thing to do in this situation is to do nothing. Let your male swan live in peace on his pond for the remainder of his life.
Male swans typically do not re-pair with another female swan, especially after having spent most of their lives with a previous mate. This is an individual attribute, but usually holds true with most male swans.
The problems with introducing another mate besides him not accepting it are:
1. His age. You do not know when nature will take its course and he is at an age that makes it harder to determine exactly how many more years he might have. So, if you get him a mate and he doesn’t live much longer, you will be right back where you are now trying to find another mate for the female and hope they pair. His fertility will certainly be affected by age so there are no guarantees that he will be able to produce cygnets.
There are U.S. laws regarding the breeding of Mute Swans as federal and state wildlife officials have been trying to kill the species for years. Wildlife officials want to kill this species to free up habitat to introduce another species, Trumpeter Swans for Trophy Waterfowl hunting. Therefore, you may or may not be able to import a mute swan without a permit or breed the swans without a permit. You will need to have all cygnets pinioned, permanently rendered unable to fly by a licensed veterinarian at 1-3 weeks of age. You will also be responsible for all cygnets including finding a new home for them as fights will ensue every year during mating season. Mute Swans can have 1-8 cygnets a year which means you will eventually need to find a new home for many of them. All of this may mean that you become a breeder under the law requiring a permit and other regulations.
2. You will need to build a pen, enclosed top to bottom (so no predators can climb into or dig under the pen–must be half in water and half on land with zero entrance into pen–no steep banks, must have feeder inside) so that the new mate can get accustomed to the male swan, you and the new environment. She must stay in this pen for approximately 3-6 weeks to ensure there is no aggressiveness on the part of either swan, that she does not wander and get herself killed because she is not familiar with the new environment and the presence of predators. No matter if you get new swans, they will still have to remain in a pen to acclimate them to you, the male swan and the habitat.
3. If you introduce a female or another pair of swans, you risk them being chased by him from the habitat and into a predator’s area.
4. If the new female does not get along with him, she could chase him out of the habitat because she will be stronger and younger than him. If he were to prevail, he could seriously injure or kill her.
5. If you get another pair, during mating season, there will be fights and he could be seriously injured or killed by the new swans as they will be stronger than him. In nature, strength prevails and he will be displaced.
You are only setting up your male swan to much stress by introducing any new swan. Your pond is and has been his home and it is not fair to him to have to defend it every day against new swans or have him displaced. You certainly do not want to introduce another male as a pal because one or both will be injured or killed.
The best thing to do is just let him enjoy his habitat free from stress and you enjoy him through his older years. I hope this information is of benefit. The Regal Swan