@Xxjuggernaut I don't think you understand. The mangaka isn't trying to write the "smartest" possible action.
Tetsuo does everything for the sake of his family, it was only once his family was mentioned here that he really took action. In fact, he wouldn't have done or have been capable of half the stuff in this manga if he didn't care for his family so much. He's not a big manly man, he just a regular salaryman who loves his family.
That's his greatest strength.
But it's also his greatest weakness.
Despite the fact that this guy is Nobuto's father, he still identifies with him as a father as well.
It doesn't matter if Nobuto's a scumbag. He knows that he'd love him anyway.
I'm also not 100% sure that Tetsuo even knows exactly what Nobuto's dad's gang connections are, he seemed incredibly confused that he wouldn't just call the police.
If you can't understand why Tetsuo finds it harder to frame someone from the syndicate than he does to act against this guy, I'm not sure you really understand Tetsuo's character.
This manga would be 1000 times more boring if Tetsuo was this hardened remorseless killer or invincible hero of justice. He's just a regular, rather meek, guy who cares a lot for his family.