Let's be honest. It's probably his kid. She probably got pregnant and never told him because at the time he was focused on... something, probably, and she didn't want to burden him with it and naively thought it's better to raise the kid on her own.
As much of a brown(?) flag the main guy is, I appreciate how nonchalantly this manga approaches romance. MC's also on the assertive side, which is refreshing. Romances with adult characters really are the best.