"You are going to be a dad"

So I'm 3 almost 4 weeks pregnant... I'm 16 going to be 17 on Halloween. I'm pregnant for this guy who's like 21 or whatever. He has a girlfriend i'm sure. My mom says it's smart to wait until you are done with your first trimester before announcing you're pregnant. I'm terrified to tell him... I'm scared. When do you think is the right time to tell somebody you hardly know and who've you only got with one time that they're going to be a father? What are the right words to say? Should I just tell him over Facebook? Please comment😔😓