Just "Mister". As for his rank emblem, it appears to be a plain metal square - no markings on it at all.
Given his demeanour I doubt he was a WO or similar, the uniform notwithstanding. They may not be very military, but neither are they so lax that a random expert of the week would get away with gross insubordination. My guess would be that he's a civvie hired for his expertise, and wearing Starfleet uniform because of some obscure regulation.