The time to have made your nephew face consequences would have been when the availability of saying "NO" to co-signing the loan was an option. Co-signing loans and lending money are almost always bad options outside of the commercial world. Co-signing a loan for someone else amounts to signing the loan yourself because when the other signer does not pay for any reason or no reason, you are primarily liable on the loan. The lending institution can proceed against either signer. As co-signer you can sue the co-signer and get a judgment. Then, you fact the prospect of collecting on the judgment.
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This answer is provided for informational purposes only. Actual legal advice can only be provided in an office consultation by an attorney licensed in your jurisdiction, with experience in the area of law in which your concern lies.
When deciding if you should sue someone there are two essential requirements. First, did the person who you think violated your rights have a duty to refrain from the activity that you think would form the basis of a suit or did they have a legal duty to do something and they did not do it. It is very difficult to determine the answer to that question based on the facts you list because it will depend on State law and possibly administrative law in your State and under Federal Law.
The second essential is where there compensable damages? Damages for which a Court can award you monetary awards or injunctive relief (order the other person to do something or stop doing something). If you have both of these elements you may sue.
However, lawsuits take a high degree of expertise and cost money. Many clients have come to me through the years and stated that the money did not matter to them, just the principal of the issue! When I tell them how much I and other lawyers charge by the hour it becomes obvious to them that the value of the lawsuit damages is very important.
You can seek out your lawyer referral service to seek counsel. There are agencies of the State and Federal Government you may want to contact. Talk to a lawyer before you decide to sue someone for expert legal advice!
Disclaimer: This answer does not constitute legal advice. I am admitted in the States of New York, New Jersey and Massachusetts only and make no attempt to opine on matters of law that are not relevant to those three States. This advice is based on general principles of law that may or may not relate to your specific situation. Facts and laws change and these possible changes will affect the advice provided here. Consult an attorney in your locale before you act on any of this advice. You should not rely on this advice alone and nothing in these communications creates an attorney client relationship. The opinions expressed herein are those of the author only and the fact that he has worked as an Assistant District Attorney; State Supreme Court Clerk; Special Assistant United States Attorney (Hawaii); Assistant Cornell University Counsel or Judge Advocate, United States Marine Corps should not be relied upon to assume that these statements reflect the policy of these organizations.
He claims it was a gift. You claim it was a loan, but you have no documentation of it, other than the check that you gave him that may have some language that may lend support to your loan argument. Also, his two payments support your position it was a loan and the fact that he only stopped paying was because he lost his job. Also, you may have family members who can testify, if you sue him in small claims court.
I doubt you can claim the vehicle as yours, as you don't have a lien on it filed with the DMV. Rather than sue him, why don't you ask him if he would agree to you having a lien on the vehicle with the DMV (which will further document that this was a loan from you) and then have a written agreement that when he gets a job, he will begin making payments to you for $x amount until paid in full, so long as he remains employed. This will also protect you from his taking the vehicle to a title pawn lender and borrowing an addition thousand or two against the vehicle. Title loans are expensive and often result in repossession and bad credit.