I restore cars. I know what that guy goes through. He was an idiot for signing the contract and he should honor it. That isn't the point.
The point is this: "The squeekiest wheel gets the most grease!" I use this tactic all the time. I would call or stop by every day, maybe with a smile or a scoul, but everyday. " How's my truck? Is it done yet? When will it be? I will call you tomorrow to see what the progress is!" It is cheap and not all that time consuming. Be cheerful and act like an annoying idiot. He will get sick of it soon. He will either give you your truck back or finish it. Either way he will get real tired of your annoying distractions. A lawyer will piss him off.
Not to sound mean, but did you really believe him when he signed that contract? I personally am too cynical and would have laughed at him.
By the way, my boss has a Mercedes that he took in 15 years ago and he still hasn't even touched it yet. ( The owner gets free storage and doesn't care)