Continuity mistake: When Brian spends his first night in the underground monster world he goes in wearing a shirt and boxer shorts. But a later scene shows him wearing a shirt, pants, and shoes. He then goes back home and is back in his shirt and boxer shorts.
Suggested correction: It's entirely possible that he was able to borrow a pair of pants while down there.
I just watched the scene where Maurice and Brian pull the pranks to get the kids into trouble and he is in his pyjamas. Later in the monster world he is in a full outfit including a shirt, pants, and shoes. Then he goes back home and he is in his pyjamas again.