When I was applying I had a few 'informal chats' with professors and then full blown interviews with a panel of people. This sounds like he just wants to get a feel of what you are like, what your motivations are etc. I wore a suit to all of my meetings with potential supervisors. You are never going to be make a bad impression in a suit to an interview, and you can always take your tie or jacket off to dress down. My partner did a masters at cambridge and I got the impression that as a university they love pomp and ceremony, so a suit wouldn't look out of place!