Are you absolutely sure he is doing this? We were in similar circs quite a few years ago, were very worried about it and so paid for some initial legal advice. The solicitor laughed (nicely) and asked “Is he filthy rich?”
He told us that bringing a case is very expensive and solicitors and barristers generally won’t take them on unless they are sure they will get paid. No win no fee-ers generally won’t even think about touching them. Unless the case is against a paper, where the paper can be made to print a retraction or apology AND to cough up; or against someone who themselves is minted, there is no point in doing it. If the person you sue has no money then the plaintiff won’t get money (and may not be awarded any anyway) and won’t get costs even if they are awarded, which they may not be. He will end up paying dearly for his fun.
In our case we did not respond to the letters or emails he sent and when he turned up at our door shouting the odds we told him to leave or we would call the police.