Booster and Rips' first on panel meeting was in the first Booster Gold series. Rip helped save Booster and bring Michelle back to the past but while there he learned some bad things were coming before the 25th century. Nuclear wars to be exact, and that knowledge haunted him. Rip made a desperate attempt to stop such a disaster from taking place. Knowing he needed help Rip asked Booster to inform the superhero community. Some weren't sure how to help, some questioned whether time travel should be messed with, etc. Booster even helped rounding up someone he really didn't get along with at the time--Superman.
So to recap: Superman basically mocks Rips' plea for help. He doesn't even ask to look at this proof Rip says he has and tells him to call the league instead (the same league the guy that told him--Booster--is currently on.) Superman never offers any help throughout the rest of the first Time Masters mini series (Hal even helps out) and only showed up to say he's not helping. This is one of the reasons I laughed so hard when Rip puts Superman in his place in TMVP. Because believe me, Superman deserves it when it comes to the Carters.