How do I approach the subject of fathers?

I am a child of artificial insemination and my mom is a single mother by choice so I’ve never had a father. As such, I’ve never really known how to ask about people’s families, if the situation is an odd one. Normally, this isn’t a problem, but I’ve recently started dating this guy and his father is no longer in the picture (they haven’t spoken in 7 years) but he does talk about his father sometimes (he’s mentioned vacations they’ve taken together). How do I ask him about his father without stirring up nasty feelings?