Trust me on this one. It's a poor argument. By all means, emphasise that you run a business called "thousandmonkies.com" but as soon as you start bickering that you're ".com" and he's "limited" you have already lost your case.
Simply say you have a different client base, different industry, and have been legally using the domain name since before the internet was invented.
By darting back and forth poorly thought out correspondence with lame arguments, you'll come across as an amature, and he'll put together a correspondence file, bring it to his solicitors, and say "get this domain name for me."