Liam was born sometime later after Katie and Chuck's marriage. He starts to talk and crawl, much to Max's discomfort. However he really loves his pet dogs Max and Duke, and imitates them in many of their manners; thanks to their developed friendship, Max becomes overprotective over him.
Liam goes to the trip to uncle Shep's farm, in which he interacts with Rooster as well.
At the end of the film, Liam goes to his first day of preschool, and says goodbye to Max and Duke, as well to his parents.
- According to Max, Katie says Liam's head circumference is in the 80th percentile.