His Father's Son Dr. Quinn, Medicine Woman : Season 5 Episode 24

Everyone is doting on Katie and Brian feels slighted. Brian receives a letter and picture from his real father, which leaves him feeling like he doesn't belong anywhere. Brian starts behaving badly. He gets Anthony to skip school with him and they hop a train. Sully punishes Brian. When Brian questions Sully's actions, Sully says that he is doing it because he is his Pa.