It sounds like he must have sold at least 3 cars. If he has done that in a relatively short space of time, he is clearly a dealer, not a private seller. That gives you additional rights.
With a private seller, the car must match any description that was given and be roadworthy. If, for example, the seller states that the car has air conditioning, it must have air conditioning fitted and it must work. If he is a private seller, you are probably going to have to chalk this one up to experience.
If, however, you buy from a dealer, the car must match the description, be roadworthy, be fit for purpose and be of satisfactory quality taking into account its age and mileage. From the information you have given, the car is not of satisfactory quality so you can return it and receive a full refund. However, given that this is a dealer pretending to be a private seller, this may not be easy. If you want to take him to court, you will need to prove that he is a dealer (you don't need absolute proof - this would be a civil case, so it would be decided on the balance of probabilities).
It sounds like you may already have evidence that he is a dealer, although it isn't clear if the evidence is sufficient. If you want to pursue the legal route (which is probably the only way you are going to get any money out of him, and even then it is not guaranteed), you should send him a letter before action - basically a letter setting out the facts, saying what he owes you, giving him a reasonable deadline to respond (28 days is normal) and stating that you will take legal action without further notice if he doesn't respond satisfactorily by then.