Toxic relationship

How do you get out of a toxic relationship? I’ve been with my significant other for the past 7 years. Through out those years we’ve been on and off a lot for many different reasons and a lot of it was him cheating on me but stupid me I’ve always put it in the past and continued our relationship. We have a 6 month together and I always have to remind him that if our issues begin to involve our son in any sort of way that I’m taking our son and we can share custody. Every time I try to have a serious conversation about it he shoves it aside because he’s said in the past he’s only here for his son and nothing else. He’s going to deal with me just until our son is 18 because he wants to be able to see him everyday and not on a set schedule. He’s verbally and mentally abusive to me and my son is going to get to the point where he knows what’s going on and think it’s okay to act that way. It’s absolutely wrong !! It sucks that regardless if I’m nice to him even when I shouldn’t be he treats me like complete shit . He has to stomp on every single piece of happiness I have left . I’m fed up ! I wish i had a way to feel better I’m sitting here crying my eyes out in a shitty mood this Friday night ...