If the juvenile is a male, it will probably not pair up with the older male swan. A lot of defensive behavior is occurring at this time of the year due to mating and nesting season. It would be better to leave the juvenile on the side of the pond that he departed so that the possibility of getting beaten has been mitigated. If he feels secure later, he may move towards the other swan, but again, it would be better if he does this by himself instead of human intervention. The Regal Swan