To divorce or stay together?

We have been married 7 years. My husband is not exactly mentally stable. I truly think he needs help but he refuses to do so. We have a son who's almost 5. He is very much a mamas boy. I also do 90% of his care. My husband also has an 11 yr old daughter we have every other weekend. She is a bit of a handful (i think every girl can be at that age). He constantly yells at her and they fight everyday she is over. I think he should keep his cool as the adult but he blames her for making him mad. He has started really yelling at our son too. When i call him out, he tells me i shouldnt be telling him how to parent. His daughter has, in tears, told him that at this point she doesn't even want to come over anymore and fears not wanting a relationship with her dad as an adult. He brushed her off and told her she needs to get herself together and stop provoking him and that she may have a mental illness like her mom (who as far as i know has only been on meds for anxiety). His response to her heartfelt speech made me sad. I worry if we were to divorce, and he gets 50/50, that it would be a unstable environment for my son and i wouldn't be there to protect him. He has already stated when we talked about divorce before that he would go for 50/50. If we did not have a child together, i would have left already. He is emotionally abusive and selfish and possibly narcissistic. Is it better to stay knowing I'm always there for my son? Or to leave, show him a happy home and hope for the best when he is at his dads?