If this was a private sale, the only obligation on the seller is that the goods must be as described. The relevant law is the Sale of Goods Act. This applies only if you were both consumers, and you were not making a trade sale (you say you were selling “my car” so it sounds like this should be the case).
If the vehicle stopped working after you sold it to him, it is for him to prove that you knew or should have known about the faults prior to the sale, and therefore that when you sold the vehicle to him, you effectively withheld that information.
The only way he can do this is by production of an expert engineering report which states that:
The vehicle was faulty:
The faults were “x,y,z” (all faults must be described in the report):
It is more likely than not that the faults were known to you before the sale.
Even then, in addition to the above, he must prove that the description given of the vehicle by you (either verbally or advertised) differed from the reality. It’s quite a difficult thing to prove. In essence, unless there was something obviously wrong with the car, and the engineering evidence he produces proves you probably knew about it and you misled him, he is likely to lose.
It sounds like he hasn’t obtained engineering evidence so his claim will not succeed without it.
From what you say, it sounds like this may be ripe for a strike out application.
Dean