The main thing Tony remembers about Howard as a father is his absence.
Howard was much too busy with his company, and Tony wasn't really fathered much as a result. By the time of Endgame, Tony has seen the video Howard left for him, so he knows that Howard did in some way notice Tony, but Howard is still a very incomplete image from Tony's past. His business acumen is well documented, but Tony never really knew Howard as a father.
To me, this scene reads like Tony is continuing the conversation about Howard's child, in an attempt to be able to read Howard's feelings about being a father. Was he excited about it? Was he disinterested? Was he dreading the thought?
This could help reframe Tony's opinion on Howard. Maybe Howard never was interested in children. Maybe he was, but the company unexpectedly took up more of his time than he was anticipating. In this conversation, Tony has an opportunity to maybe find that out.
Tony has a limited window on getting to know his father face-to-face (i.e. this specific conversation), so he's trying to get a read on Howard by continuing the topic of Howard being Tony's father (unbeknownst to Howard at the time that he is speaking to Tony).