He said he doesn't care what the old man thinks, he can talk all the shit he wants, doesn't mean tsugumi has to defend himself, he already has it clear that he's not all those bad things megumi's dad is saying. Plus he's doing the smart thing, having a bad relationship with megumi's dad wouldn't be convenient for him
Well tsugumi understood that megumi's dad is judging him based on what he saw him do and or how he acted in his presence, so he concluded the only way to change his mind is to show him that he's different and not all that bad, show him his good qualities. So instead of pointlessly trying to convince him that he's not a bad guy using words, he'll show him with his actions
To be honest, I’m little disappointed at how tsugumi is accepting all these downgrading Words…. He could just confront and accept the confrontation but he’s just sitting there and accepting…. I don’t see the logic in it