It is unacceptable to tease someone for making a mistake.
Tony's son leaves his bike out in the rain and it rusts.
Tony wants this son to realize the mistake he made.
Tony talks to his son about the importance of taking care of things and tells him he'll need to help pay to repair his bike.
Tony makes fun of his son for leaving his bike out and laughs about how his son can't use it anymore.
Tony realizes that he'd better be more responsible for his belongings since they aren't free.
Tony's son feels stupid for his mistake but also hurt by his father laughing at him.