I'm going to offer a slightly different view of things, based on my experiences with Small Claims Courts. Realize that small claims court may operate differently from state to state. If he is, indeed, going to file in small claims court, then you owe him no additional information. I would wait and see if he actually files in court. You will be notified when he does, as the defendant. You may not need a lawyer to defend yourself. He is required to detail his losses and you can challenge them in front of the judge. Merely saying there are $2500 in cosmetic damages does not cut it. He must detail exactly what damages he had repaired or provide estimates of what he needs to repair. You can also have your son and/or the other guys son testify in front of the judge to establish exactly where the boys got the liquor and detail all their activities that night. Based on what I have read here, you may be able to establish that the liability for the problems that occurred on your property is joint. His part is that the kids got access to the liquor cabinet. Your part is you didn't secure the keys to the tractor, and damages resulting from the boys' use of liquor occurred with your tractor on both pieces of property, with your son at the wheel. Ultimately the judge will determine a percent of liability and award damages accordingly.
At this point, until you are served with a notice of lawsuit (whether it is small claims or a regular civil claim), I would continue a dialogue with him (if possible, and keep pointing out that his son and his liquor was involved) and make him make his move. You don't owe him any insurance information or any other information, for that matter. Lots of people talk and only a few carry through with their lawsuit threats. After seeing the contents of his lawsuit, I would then decide to hire a lawyer or not. I would, for my own protection, get a copy of the incident report. That will be the official account of the incident and basis for other future actions.
Good luck as you navigate through this. Keep a clear head, be as objective as you can about the situation, and retain an even temperament....Roy