If someone is keeping you away from your friends and being a jerk to you what should you do?

A few weeks ago I started talking to a girl Kellie that my friend steve used to like. We were just friends, but Steve automaticly umed I was hitting on her with no proof and started acting like a jerk. He was one of my best friends and now he refuses to be my friend for no good reason. Me and Kellie are dating now because we figured that if steve is going to act like a little girl and hold a ridiculous grudge over somthing as stupid as this then he is not a friend worth having, but the problem is that now steve is being a jerk and keeping me away from my other friends by making me feel unwanted because he knows that I don't like being anywhere where someone makes me feel unwanted. None of my other friends have a problem with me its just him. I've totally hit a wall and I have no idea how to go about solving this.