You live in a world of jean shorts and free t-shirts from whatever convention you last went to.
He lives in a world of high finance and suits bought on Saville Row.
Interviewee should have understood expectations going in. We had a kid interview here several years ago that was wearing a short sleeve shirt underneath his suit, and then took of the jacket while in one of the senior partner's office. Automatic strike against the kid.
You have to dress the part. That is true in any industry.