This unnamed boy and later man is Penny and Leonard's nephew, so he is possibly the son of Lisa and her husband, and also the nephew of Randall. He has never appeared on the show, but has been mentioned a couple of times.
Penny looking for a comic book for her nephew.
It was revealed in "The Hofstadter Isotope" (S02E20) that he is 13 years old in 2009. Penny wants to buy him a comic book and follows the gang to the comic book store. She buys the comics recommended by Stuart.
Her father Wyatt did mention that he didn't want his grandchildren to live in a house with wheels, so does his grandson live in a mobile home?