I'd suggest you write a brief letter to him, reminding him of the fact that you'd informed him the day previously that he was significantly closer to the boundary than authorised, that you've reported this to the compliance department and, if necessary, will now place the matter in the hands of your legal advisors.
He'll probably pay as much attention to your letter as he did to your conversation, but you'll have notified all parties in writing and the paper trail will be there for the record. If you're talking to him again, you could point out to him that it'll cost him a lot more to put things right later.