Boyfriends son..

I have to watch my boyfriend's son by myself for about 9-10 hours everyday and I'm with him 24/7 just about. I catch myself being less patient and yelling more at him because he just does NOT listen. I'm afraid at times that his bad behavior is going to rub off onto my child that I'm having and I feel that thinking that makes me even more angry and nervous. Is anyone else experiencing this? This child stresses me out everyday because he is so bad! Someone please help, before I go insane! Sometimes I feel like I'm going to have a miscarriage because I'm around this negative energy all of the time. I'm just so stressed over this one 4 year old! Advice would be so encouraging and God sent at this point, haha.... but seriously.