My Son Won't Stop Biting!

Jas • 2017 will be a better year! Working to make my hubby and son proud! #72915
My son just turned one and he bites like crazy. He's even left bruises on me and little teeth mark. It's gotten to the point where I don't let anyone but my husband and grandma hold him because of his biting. 
How do I get a 1-year old to stop with the biting? Why does he bite anyway? (I'm a FTM).