shhh basic knowledge doesn't work in manga
Not to sound condescending but, he literally doesn't know how to rely on other people because he never had someone to lean on before. So his first thought wouldn't be to get help, but to try to solve it on his own like he'd been doing before.
When considering the reality of the situation, both police officers like teachers typically view children's fights as minor disputes and often don't intervene significantly. In truth, he would likely face more consequences than those he reports. He lacks people at school to support his claims and vouch for him, so in the event of a thorough investigation, few would defend his innocence. Moreover, involving the police might exacerbate the situation. Those bullies are not the type to retreat, even when law enforcement is involved.In addition as ravenapplesauce replied, he never had a significant adult to rely on not everyone can confide in another if they never had someone in their life who could've guided them in dismal times, naturally it'll be hard to advance out of his shell. Hope this gives you an idea.
This is so fucking stupid, he literally could've just called the police.
He could've avoided fighting, and it's not like Kitahara would've gotten in trouble for being literally abducted!!!!
Meanwhile this dumbass is like "woe is me, guess I have to fight, can't have friends after all... :("