"Family" is so much more than sharing a roof or a name.

Your family is broken as it is.

You are in love with someone else, your husband is an abusive control freak, your FIL is an obnoxious drunk and your poor child is witnessing all of this thinking its all normal.

What you describe doesn't sound like a family unit at all. Sounds more like random people thrown together by circumstance.

Your son has a much better chance of growing up healthy and wise, if he gets to grow up watching his parents parent, even though they realized they could no longer marriage.

He has a much higher chance of being mentally scarred from watching his parents grow more and more resentful and abusive toward each other (verbal and physical altercations ) knowing in the end the stayed together for him, and ultimately blaming himself for their misery...

He also has a much higher chance of emulating this negative behavior, as he will most likely grow up to treat a woman the way his father treated his mother... Children learn what they live.

Or he could watch you part ways and learn that no matter how hard it is to start over, we can find the strength and courage to dust ourselves off and carve out a happy and healthy new existence for ourselves. Because we deserve to live with joy, love, dignity and self respect.

No judgment here, just sharing my perspective from one mother to another.