Agree with others about no bother with Court action. I have a bit of experience and it would be very difficult to gain judgement in your favour. All the defendant has to prove is they have little mechanical knowledge and can get away with nearly anything. It would depend on a number of factors though such as how long the seller owned the car, how much they drove it, knowledge etc. I of course do not know the full situation but you would need deep pockets.
I appreciate your comments and feel sorry for your lad. An expensive lesson.