I would call the police and ask them what the best course of action would be. If he shows up sober what should you do? If he shows up intoxicated or under the influence of something should you do something different? Is it legal to have plates on the car or should they be surrendered - and is it legal for you to surrender them? I know that in one state it is illegal to take the plates off of a car you do not own unless you are an officer of the law - and the penalty is pretty steep. If someone owns a car and another person removes the plates for any reason, they can be charged, fined, etc...
Mad enough at you, or bored and wanting to see you upset, it may be something your child has you charged with. Esp if you "turn him in" in his mind. Yeah, you could probably argue your way out of it, but do you want to deal with it? If he claims the car was damaged while in your possession/parked on your property he could file a claim against your homeowner's insurance saying that it was vandalized while in your possession.
Instead ask the cops what you should do. If the car is on your property and is unlicensed or has tags that are not legal because the registered owner does not have insurance, what are your options? Can you file a lien against the car's value as a storage fee? Can you have it towed by the police as a car that is parked on your property with-o permission or past the time when permission expired?
Can you refuse to give him the keys or give them to the cops to have him get the keys from them? If you know when he will show up to get the keys, can an officer be present at your home to keep the peace? If there is a loan on the car, can you give the keys to the loan company because the car is uninsured/unregistered?
You probably have the most options as the owner of the land the car is parked on. As he has not moved the car in X amount of time, can it be considered abandoned? Can it be towed as abandoned? Can the police tow it because you don't want it on your property any longer and know the owner has let the insurance lapse? You can say that you do not want an uninsured vehicle on your property and cannot return it to its owner as he is unlicensed, uninsured and the car is not currently registered. That may be the strongest option. when we lived in an apartment, the management would insist cars be moved every 24 or 48 hours. I was a stay at home mom and often walked to places in the neighborhood so I didn't always drive my car every day. We had fits with the manager because I was "hogging" a "prime" space, meaning one in front of my apartment. So I would have to move it to another space every day, which annoyed the snot out of me. If I didn't they could have it towed. Private towing companies will tow the car and then not let the owner have it until the towing fees are paid. If the owner wants to drive it away they must show license, insurance and current registration or the lot owners were not allowed to return the car.
If the car is not driveable, say it won't start or some part "disappeared", it could go one of several ways. You could have it towed from your property as junk and he would have to pay to get it back. You could also have him sue you claiming it was damaged while it was in your care.
I am mentioning these options and suggesting you find out what your LEGAL options are because your difficult child strikes me as possibly manipulative enough to attempt to sue you to force you to support him. If he has found backers to fund his poker playing, he is likely sophisticated enough to find a sleazy attorney to help him force you to give him money in exchange for not suing you, or to have the attorney actually sue you.
You have time, so if at all possible find out what your legal options are and use them to keep your son from driving and push him into a situation where he has to have license, registration and insurance - plus all parking and traffic tickets paid up - before he can reclaim the car. He will be angry with you, but he won't be able to take legal action against you.
You probably could get away with taking the plates or whatever, but you open yourself up to the possibility of legal/insurance hassles from your son. If he devotes much time to scamming people, this is not an uncommon insurance scam - park the car at someone's house, leave it a couple of days, have parts/plates disappear and get a check from the homeowner or file and insurance claim - or both if you can get cash from them!