Book about a loser that becomes confident after finding his girlfriend in bed with his boss

[Hanne]

It was a book I read over 15 years ago. It was translated into Dutch and had a black cover with underwear that had an ‘s’ on it. I thought it was called something like ‘Superman’ but I can’t find it anywhere.
It was about a man that was a bit too naive, a bit of a nerd. He came home one day in his appartment and found his girlfriend in bed with his boss. I remember he threw up. It was something like the boss even finished after he saw the man. And then was this huge transformation where in the end he became this handsome succesfilm guy and the girlfriend wanted him back. . .