one thing I see so far, is there a 3rd person there when you give him the money for deposit? or did you have a recipe said he took the money off you and he signed?
2nd does the name of the registation is same person who took your deposit money? if not the same you can sue whoever took the money by selling something not belong to him. that is illegal, like I sold the company I work for good amount of money but my boss don't even know?
3rd if you decide to let lawyer to deal with the car, you only need to tell him or his mom, "my lawyer going to handle it, any more question, talk to my lawyer." that will give them something to worry about it, even you did or did not let the lawyer handle it.( but in their end they knows is something wrong because is the sub removal cause the problem so as the car still at their drive way.)
4th, if they did sign over the car to you when he took the deposit, well basically you are the owner of the car, and the car end up is on his driveway, you can just call the police said he stole your car by use the spare keys