Heinlein's book, Red Planet, is about a boy: Jim, and his basketball-shaped alien pet/friend: Willis. Reading it gave me deja-vu. I know these characters from another tale; maybe not by the same names or author. In the other story, tha alien outlives the human. At the end, he's talking to his descendents, saying: "Yes. I knew your grandfather. He was my best friend!". Please tell me the story name and/or author.