SpongeBob's grandson appears only at the end of the episode, where he is being told the story of where SpongeBob and Patrick went on a quest to find the formula (see the episode for more information). However, he is not interested in this story and cares more for his video game.
His figure resembles SpongeBob, but he has a slightly different outfit, including a red cap. He doesn't wear a belt. He also wears a red tie that is similar to a "Kentucky bow tie" on his pants.
"Shh! I just beat my high score! Oh Yeah!" (first and only line)
This character shows that SpongeBob will in fact marry and had a kid(s), who in turn had a kid(s) of his or her own.
The age of the grandson is not known. However, he cannot be older than 63 years old, assuming that Sponges (i.e. SpongeBob's child) leave home at about the age of 12 years like SpongeBob most likely did.
SpongeBob's Grandson face is really similar to the face of SpongeBob.
The handheld console he is playing is similar the gameboy.