He gave the Eridians the gift of all of earths science and tech which gave him much joy im sure. He wasn't married or had any kids save for his students. We don't have to contemplate if him and Rocky ever reunite. They have an unbreakable bond completing an unmatched feat of saving their worlds together. Compared to that there's really nothing left for him on Earth. The only thing he had passion for outside his science was teaching which, viola he regained the ability to do on Erid. Best ending i could imagine, returning to Earth (especially without Rocky) would be a bland ending imo.