Telling ex I'm pregnant?

Was just contacted by a guy I was dating 6-8 weeks before conception. Need to tell him I'm pregnant. Just "catching up" and he will see it on social media anyway. Worried he'll freak out and think it's his. How do I break it gently to him and make SURE he knows there's no doubt of the paternity?

Edit: no, definitely not the father. We had sex one time eight weeks and two periods before I conceived.