Stuck.

Just looking for some words of wisdom. 
My SO and I are engaged. He has a 3 1/2 year old son that calls me mommy. His son is my whole world. I love him more than anything. Lately everytime we argue he throws a way in to remind me his son is his. In no way mine. I want to leave him soooo bad for all the hell he has put me through. But how do I leave that amazing beautiful baby boy?