My personal guideline in these disputes is to ask the player to describe why the power would work the way he thinks, speaking in-game instead of by rules references. If he can't, then I go ahead and rule the way I was going to. So I would have asked, "the dragon is pinned even though you can't move? What are you doing that pins it?" And if he gave me something plausible, I'd go with it. If not, I'd say that he had pinned the dragon by fighting with it and that he can't now that he's unable to act.