You said in one part that your b/f gave the guy the keys to fix the car and in another part, the tow truck driver just gave the car to him. Which one is it?
Not to sound like a ***, but if you told me this story in person, I would tell you that either your b/f is not telling you the whole story or he is lying. The story doesn't make sense. Why in the world would someone "lose" a car almost a year ago and not only keep paying for it like they are driving it, but not inform anyone that they suspect it stolen. It just doesn't sound totally legit and you might want to pry some more out of your bf.
If this is really what happened, you are going to have an ordeal ahead of you, because the police, the dealership, and most importantly the insurance company are all going to have the same reaction, stuff like this doesnt happen. You first will have to make a police report after you gather infomation on the car like the address it was stored at, who picked it up from the tow truck company, who cashed the checks that you wrote for the bodywork.
I would doubt your boyfriend will be arrested, unless of course he fucked up so bad, came up with this story, and is prepared to file a false police report to try to sort it all out.
Again, not trying to be a *** but get used to this reaction cause you are going to be seeing it a lot. Good Luck and try to post more details cause as the story stands right now, I think you b/f is full of crap.