Wow, you sure got a lot of responses! Mine's a little different, though. The fact is, you did buy the car, so no sense rehashing that. The question is, did you buy it for the family and are allowing him to drive, or did you buy it for him? If you bought it for him, the truly responsible thing to do is to give him the title to it, and let him sell it on his own & buy something else he wants. Learning how that whole process works will give him a real sense of responsibility & can be a great learning experience.
If you bought the car for the family, you could decide on a dollar amount for a new car and can either just buy the new car & keep the Mustang & continue restoring, or you can sell it as-is and buy a new car with some of the profits. (But the agreed-upon amount for a new car should be close to what you paid for the car before restoring, unless he's put a lot of muscle into the restoring, too, which in that case, he deserves to profit from his hard work)
Most of the posts are telling you your son is spoiled, and blah blah blah. I don't care about any of that - I learned a long time ago never to pass judgment on someone else's parenting skills. So really, the question you need answered is what to do now so that all parties are okay with the decision. You have already purchased a car for your son, and it's okay for him not to like it anymore. That's life. Heck, I buy things all the time I don't like - and that's included a car! But since he is young, he can't just go out and rectify the situation on his own. Sure, you could say "hey, tough luck. live with the consequences" but this doesn't really teach him anything, does it? It would be better to teach him how to be an adult about it - either the title in his name & he sells, or agree on a dollar amount for a new one & you decide whether to keep or sell it. Treat him like an adult and sit down and discuss the situation. Ask him to come up with a few solutions that would benefit everyone.
I don't necessarily think he is acting spoiled. As a teenager, you don't have a good grasp of what it's like to be an adult, and that includes money situations like buying cars. You already bought him a car, so in his mind, why is it a big deal to sell it and get something else? It's not, really, it just seems like it's a big deal because you've put so much time and money into it. So instead of remaining on opposite sides, come together, acknowledge what's been put into the car already, decide to keep or sell, and decide on some money guidelines.
Good luck!!
S.