If sold as seen, then that is it.
It was not sold 'in working order', so he has no case.
And after 90 days, who knows he might have chipped the engine, blown the engine, and put the old chip back.
All you have to do is say 'sold as seen', and then ignore any further correspondance.
He can threaten litigation, but the courts would not entertain his claim.
He can squeal, he can send a solicitors letter, but remember a solicitors letter is not the paper it is written on.
If he sends a solicitors letter, and I think he won't, then reply in such a way that it says nothing but raises questions.
For example 'I don't understand your compliant, please explain further'.
But reply to his solicitor, do this yourself.
His first solicitors letter will be free, or minimum cost. If his solicitor replies to correspndance, there will be a bill. So if the guy is an idiot you can run up his bill, a lot.
The guy is an idiot.
Don't even get involved in a dialogue with him directly.
All you have to do is say 'sold as seen', and then ignore any further correspondance.
If he persists, tell him you have taken legal advice and if he continues to persist say you will take legal action against him for nuisance if he does not go away.
But I suspect as soon as he knows you will fight it he will go away.