Season 6: Episode 11: Former Life of Brian
Brian, afraid that he may have lost his last chance at love, searches for a former flame, Tracy. To his surprise, he discovers that he is the father to her son, Dylan. Tracy decides that Brian should raise the boy, and she drops him off at the Griffin's house. When Dylan runs rough-shot in the house, Brian's attitude towards parenting doesn't settle too well with Lois and Peter.