Justin continues his redemption arc by going to rehab and getting clean. He puts effort into staying clean and applying for colleges. He's also still in love with Jessica, but she ends up dating Diego, another member of the football team.
After Justin's mom is found dead, seemingly due to drugs, Justin spirals and starts using again. Jessica tries to help him, but he pushes her away. However, he ends up going to Clay's parents to tell them he's been using but is trying to be better. He even removes himself from troublesome situations, including prom. At the last minute, though, he shows up at the dance in his tux to tell Jessica he loves her more than life. They dance among their friends, until Justin passes out.
He's rushed to the hospital, where a doctor tells the Jensens that as a result of his intravenous drug use and sex work, he's contracted HIV I, which has turned into AIDS. He dies in the hospital surrounded by friends and family, just before graduation.