It's a tux, just not a traditional one. You're thinking cummerbund and bowties, like Justin was dressed. Think about the Oscars, where the men wear various styles of tuxes. And actually, Brian had the bowtie, he just didn't tie it.

Also, Brian would definitely have a tux, if not two. He was a senior ad exec, and entertained clients all the time. Also, he attended the Clio Awards (remember 120?), which is a big deal in advertising.