Hmm... i think the dad is frustrated too because he doesn't know what to do to keep his family so it's a bit bad memory to him to be talked about. Relationship is indeed hard. But he chose to give up by himself so the regret would gonna last to him for whatever long until he made it clear to his former family
My own dad remarried with my stepmom when I was young (still together to this day), so I can't remember how awkward and new it was getting to know her, but I can only imagine how complicated it is knowing your parents are loving someone else who isn't part of the family, especially when you're going through your own complications of growing up (puberty, relationships, the gist). What happens after divorces happen
very differently for everyone, just as how love is, but I think what's important here is whether the child and divorced parent still see each other as family would depend on the strength of their mutual feelings and how willing they are to reach out to each other. How much are they willing to open up about the truth that led to the separation of their family, and why do the separated child/parent still matter to them despite that?
The dad may not love his ex-wife anymore, but he still cares for his own son and wants to be a part of his life, to some extent. Whether that feeling has also died down at this moment, we can only wait to see, but with how quickly he tried bailing on the conversation when the wife/mother came up, despite how much Maki wanted to know the truth, things honestly don't look good for the dad.