SpongeBob's Grandson is, as the name says, SpongeBob's Grandson. He just appears in the episode The Great Patty Caper, in which SpongeBob retells him the story of what happened, although his grandson isn't listening, because he was playing a video game. Everything happens 75 years after The Great Patty Caper. His name is SpongeBob Squarepants III.
SpongeBob's Grandson looks just like him when he was a lot younger, but he wears a red cap and tie and doesn't wear a belt. He lives in the near future.