An air horn may work, but there are two issues.

1. Don't scare the swan or he may have an issue with the horn, although he will get used to it.

2. Just like the swan, if you use it too much, the geese may get accustomed to it and the noise will no longer have the desired effect of scaring them from the area. You want to alternate the time you use the horn and maybe, even alternate the location. Additionally, do not use while you are feeding the swan as the geese may eventually ignore to get to the much wanted food supply. Sheila