Louie is missing his father and wants to see him. Like a toddler, Louie learned words like DADDY, and MOMMY. That way, he could determine who was in the house at any time by pressing either button and seeing who responded. Along with these two words, Louie also learned BYE BYE in the context of people leaving the house or when someone is away.
When Louie understood that their father was not in the house, his look explained how he was feeling about daddy being gone. Louie instead went on to say MOMMY BYE BYE, indicating that Louie either believed his mother was out of the house, or that they wanted mom to leave the house so he could be with dad.
Luckily for Louie, dad only went out for a little bit and was going to be back, at which time Louie could have all the fun he wanted with him.