Aside from the dispute he has with his registrar - he (not his registrar) is a party to a contract with Nominet that obliges him to keep his contact details up to date. So if he did not get the letter from Nominet because they had the wrong details, that is his fault and no one else's. It clear from
Nominet's Registration T&Cs 4.1.
So if the word is plain English word and you had no knowledge of his business before registering it, he really should be told to take it up with his ISP for compensation (depending on their contract) and leave you alone. Be very careful with your use though...
Of course, it is easy to see why he is not happy with things!
and then
