At the end of the film, the narrator revealed that Rex gave up his pursuit of Lori and died of Lou Gehrig's disease (which John had previously wished on him) after becoming severely depressed.
Rex was a cocky overachiever in high school. His privileged status and good looks made sure that he was popular with his peers, despite his poor personality.
Rex was a full-blown narcissist. He loved talking himself up, showing off his wealth, talents, and connections and sought to sleep with attractive women, regardless of any husbands or boyfriends. He had a low opinion of John, condescending and bullying him whenever he could. For this reason, a hurt John told Rex he hopes he dies of Lou Gehrig's disease.
Even Rex himself could admit he was a jerk, but it had worked for him his whole life, so he wasn't about to stop. It was implied that he eventually had a moment of reflection of how meaningless his life really was, which would contribute to his eventual depression and later death.
- Rex is sometimes thought to be the tertiary antagonist of the first film.