Analysis

It is shown throughout the anime and manga of MHA that Pops didn't do much parenting once he adopted Kai, only doing the bare minimum like putting a roof over Kai's head and giving him clothes and food. In regards to actual parenting, we don't really see him actually be a parent. He did nothing to genuinely help the person with violent and apathetic tendencies, who blatantly exhibits multiple symptoms of multiple mental disorders issues and has since childhood.
In addition, he never taught the child that he took in from the streets (who’d clearly faced some form of mistreatment before being in his care) to not feel indebted to him for helping them. Never bothering to figure out or ask why Kai had that mindset in the first place. Never even taking the time to actually sit him down and explain things to him, try to get him any sort of actual help, never seemed to actually consider his perspective on anything. All that, and then he wondered why the child he took in grew up to be so horrifically apathetic and violent, after he’d been showcasing those behaviors for literal years, and the Boss had done nothing about it outside of a hollow reprimand. He even had the nerve to act surprised when the child he brought up in the yakuza doesn’t care much about human lives and only searches for ways to make them useful at the expense of them which was the very mindset he willfully ignored and at least semi-encouraged in Chisaki.

So my theory on why Pops did nothing about Chisaki's violent behaviors is that mental health in Japan is heavily stigmatized. Mental healthcare is limited in a few ways. Therapists do exist but there are not very many of them. The focus is almost completely drug oriented. So the people there who deal with mental illness are taught to deal with it themselves hence why Chisaki wasn't given any help for his behaviour. Pops also strikes me as an old school type of person so I wouldn't put it past him to not put Kai in therapy.