In the episode "The Kids are Alright" 3x02, Dean reunites with Lisa Braeden, a woman he had a fling with 8 to 9 years before. He discovers she has a son named Ben who is celebrating his 8th birthday. The boy greatly resembles Dean in attitude. This causes Dean to suspect the boy is his. Lisa, however, denies this several times- saying that she had a blood test done when Ben was a baby.
Of course, there is a lot of speculation as to whether or not Lisa was telling the truth.
Later, Dean reunites once again with Lisa and Ben in the episode "Swan Song" 5x22. He spends a year with them and comes to consider Ben to be his son, whether or not he's his biological father.
