I am struggling to be able to deal with my boyfriends almost 6 year old son. He won't listen to me and it's driving me insane. Help? Any suggestions or anyone who can relate?

My partner is my best friends brother and so I've been in the family for years. His son has known me all his life. He doesn't listen to me and has this stare of 'I'm going to kill you' that he really only gives to me when he doesn't like what I say or ask him to do something. I honestly hate to say it but I really can't stand his kid. Im about to have his little sister in 20 weeks and I don't know how I'm gonna be able to deal with his misbehaving and a new born at the same time. He doesn't live with us he lives with his mother and she lets him run a muck which is half the problem. Advice?