Arey he is talking about the pang he felt inside na....woh regret toh humesha hi rahega andar se that your own kid is calling someone else Papa.....that is what he's talking about that hearing Abhir calling Nav Papa used to remind him of all he had lost in this twist of kismat.
Obviously he never BEGRUDGED Nav for being Abhir's Papa....he has shown a very large heart and graciousness about that many times and happily adjusted to threesome coparenting also....but he is also honestly admitting what any father would feel....if his own son didn't consider him Papa.
Ya, that I understood his monologue, I'm talking about the court scene, where Akshara asks if he was jealous with abhinav!!! And he admits that he used to feel bad!!! I didn't like that part personally.