Does it matter if he has a child before his relationship with you? Maybe he thinks he might be the father but doesn’t have any confirmation if it. The ex may have denied it and he hasn’t asked for a paternity test or is in denial himself.
If you are going to ask him his story, start with telling him that if he does have a child it’s not going to effect your relationship so that he knows he has nothing to worry about and feels comfortable opening up.