Deeeeep sigh. Okay, I am one of four children, my mother, God rest her soul remained married to my dad. Who I love dearly, but he was an alcoholic and is bipolar (who I unfortunately inherited it from). He was mentally abusive to my mom for over 30 years, until she died. She was such a wonderful woman, she even borrowed money from her supervisor at work (that she was dating while my parents were separated) to pay for a divorce, but during one of my dad's "nice moods" talked her back into his arms.
My childhood was spent listening to my parents fight, my dad was never physically abusive to my mom, except for once much later on, but while I was younger I was always afraid one of the times that he threatened he was never coming back, that he really wasn't, leaving for school in the morning to find him passed out in our driveway from a night out drinking, you name it, it went on. Then he sobered up and we had to deal w/the skeletons in the closet of his mental illness. Being bipolar myself I realize it's really no big deal, but my dad was on & off his meds, nastier then shi* & my mom was always there making excuses for him.
I truly got to a point in my life that when my friends would cry over their parents getting divorced, I wish mine had.
That's my take on your situation and that is pretty much my scoop on what I could give you from where your child may perceive things. I'm not in their head, but I'm pretty sure that if you asked them in a few years, it wouldn't be too far off. It's a hard choice to make, but if you do leave him, leave him for you and your child, not another person. It's flattering to have someone else to care about you and pay attention to you, but you have to always keep in mind the real reason you are doing this.
If you ever need a shoulder or a friend, please consider me good for both :)