I don't see a big down side here, I would respectfully tell the father that you made a completely legal sale with the kid and you expect him to honor it. Let him know you are taping the conversation if legal in your state. Let him file if he wants to go through the trouble. Easy to file small claims but the Dad will need to take time off to show, unless the appearance is after hours. You will also have the opportunity to file a continuance (postpone) at least once....another day off for the Dad. Many people no show to court.
Be respectful in court, do not lie. An auto sale to a minor is not automatically void, just easy to void. if you can show the kid and Dad knew what they were doing, the judge may rule the sale final. Small claims is real amateur hour for plaintiff/defendant AND the judges. Often a judge will start by asking people to try again to talk and work it out in front of the bench. The judge has a ton of leeway and is not going to think deeply about the finer points of contract law. If the Dad or kid lies to the judge they are toast. Same with you.
I would say worst case, Judge voids the deal and you are in the same spot, and tell him you now need 90 days to repay. By the way, it will take at least 30-60 days to get to court in most towns. He will probably need to file in your town. If you know the mileage, firstly they should not have been driving it; (if so why?) if they did, ask judge for standard IRS cost per mile (50 cents?) for wear and tear.
I have won 2 for 2 in small claims in NY: $3,500 from a public utility for wrecking my wall, and $3,000 from a guy who did not finish a construction project. Last time I went, I was the only case; 3 other cases "no-showed."