How to tell my Boss

So yes, i'm super nervous about telling my boss. I work at a really popular nursing rehab facility where there is a lot of older adults and young college students. Well i don't know how to tell him i'm pregnant. I'm 18, which i know is young but this baby wasn't planned, it's a miracle i am pregnant and i would never wish not to be, it's just a rough timing. He's an older guy with kids himself. I'm just super nervous telling him i'm pregnant because i'm at a younger age. Is there anyone that could help or give me some advice?