I'm not surprised ... on the bright side, you still have the domain.
Legal action, in my laymans view, is a waste of time. He didn't pay for anything, and you still have the domain ... that doesn't really leave much in the way of damages to sue for.
I'm not an attorney, but from my experience, it's unlikely a court will force him to buy it - not saying that can't, but don't see it happening in this instance ... but even if a court does rules he must buy it (and/or pay damages of some sort), he likely doesn't have any money anyways, which would put you the exact same situation as now, but with more hassles to deal with and less money after paying attorneys, etc.
Ron