Grandboy is Fanboy's grandfather. He is mentioned and never seen. Grandboy probably has met Chum Chum. Also, Grandboy is not to be confused with Fanman, a superhero.
It is unknown what Granboy looks like. He probably looks sort of like Fanboy, just as an old man. He would of wore the same purple and green suit as Fanboy, and have a head with the same shape. The symbol on Fanboy's shirt would look like Granboy's symbol. Grandboy would have a purple cape, just like Fanboy.
Granboy has never actually appeared in an episode, but he was mentioned in the episode I, Fanbot. Fanboy mentioned Grandboy. He said "I think I get that from my grampa, Grandboy."