What should we do if my boyfriend’s cousin doesn’t want to give the truck back after being paid for fixing it?

Call the police and report it as stolen.

If you have proof that you paid for the repairs (receipt or check, for example) and proof that the car is yours (pink slip, license plate, which the police can easily check), you should be able to go drive it away from wherever the mechanic has it. You probably should call the local police and ask for an escort, explaining that the mechanic is wrongfully keeping it, and you need the police there to keep the peace. This should be quick and simple. Or you could just call the police and report the vehicle stolen after demanding it back from the mechanic, hopefully in writing, such as an email, for proof that he really won't return it to you. No doubt that would cause the mechanic problems, but you might get your truck back right away and you'd probably teach this fellow a lesson. You could sue him for claim and delivery and conversion, maybe fraud and other things, depending upon the facts. That would take time and money, of course, but it is what the law allows on the civil side. This would be your choice if the police aren't convinced that you've paid for all the repairs, and the mechanic is just keeping the truck because of his lien on the vehicle for unpaid work. Isn't your boyfriend able to talk sense to his cousin? That might be the easiest thing.

Sue him. Make a police report.

Call the police for theft. Hopefully your boyfriend can prove that he paid for the repairs. A legitimate repair shop has the right to retain a vehicle until the repairs are paid for.

Sue him in small claims court for return of the vehicle. Or get the title, go over to your BF's cousin and call the police to report the problem.

Call the police, or bring an action in court for claim and delivery. If the truck's value is less than $7,500.00, you can bring a claim and delivery action in Small Claims Court (also called Summary or Magistrate's Court).

Take them to small claims.

This is the sort of thing the small claims court is designed for (what I call the Judge Judy court) sue the rascal.

Call the police. If they don't help, sue the guy. He just bought a truck.