It could be that someone was mean to him or he faced a big problem that frustrated him a lot. These kinds of situations often lead to a character being angry in a comic.
The boy in the comic might be angry because he lost a competition or his favorite toy was broken. There are countless reasons that could trigger his anger.
Maybe he didn't get what he wanted. That could easily make him angry.
It could be that the story contains something that goes against the boy's values. Like if it promotes lying or cheating, and the boy, who has strong moral beliefs, gets angry at it.
Maybe he didn't get what he wanted. That could easily make a boy in a cartoon angry.
The cartoon boy could be angry because his favorite snack was out of stock. Or it could be that someone broke his favorite toy. Maybe he was scolded by his parents for something he didn't think was his fault.
Maybe he didn't get what he wanted or someone was mean to him.
Well, an angry boy comic could tell a story of his attempts to control his temper and learn to handle difficult situations. Or it could focus on his quest for self-understanding and acceptance.
Maybe he's had a bad day or someone upset him.
An angry boy in cartoons is often shown with a scowling face, clenched fists, and maybe stomping his feet.
It might be that anger doesn't solve problems. If the boy is constantly angry but then realizes that being calm and rational helps him better deal with his situation, that could be the moral. For example, if he was angry at not being able to make friends, but then by being nice, he starts to make friends.