@jodie34,
Wow, Jodie. First - I'm heartbroken to read that you, as a parent, should have to deal with such frustration. I know it's not uncommon but still, he's your son. As a father, I can't imagine ever not loving my kids regardless of their idiotic ways (leaving out extreme acts, but I digress). I'm sorry you have to endure this, though I'm sure it's just one thing in a list of episodes.
Second, don't you think he's just posturing? Is it possible he just wants you to open the house up for him to rifle through for anything else that has monetary value as well? Or is he really that desperate over a toy? It just doesn't seem like something worth that kind of aggravation unless there is some kind of alter motive. Maybe he just lives in a different world, though. One where Starwars action figures rule with ultimate power. Or maybe he hid his stash in said action figure...
As others have said, he has no case. Let him waste his time and money on a lawsuit but, unless he has legal documentation signed by you stating that you have it and will keep it for him, he has no hope of winning. Let me take that back, this is America. He has a chance if you live in California. ;-)