Several things may be occurring. If the swan is the same age as the other swans, he is exhibiting a "leader of the pack" behavior. The holding of the head under water can be either aggressive behavior or he is trying to practice "mating" behavior. Young swans will practice.

Even if you place extra feeders, he still may exhibit territorialism. Give it a couple of days and the behavior should work its way out. Having said this, if you see extremely aggressive behavior, you may just need to give him a time out by placing him in the garage, etc. The problem with this separation is that once you have removed an alpha animal, someone else is going to step to the plate and assume leadership. If it is any consolation, they usually work it out, it just takes some time and a few nipped tail feathers in the meantime. Again, if it comes to the point that a serious injury is going to occur and simple tail nipping is not the case, you do need to separate. Good luck. Please let us know how this situation unfolds. The Regal Swan