is it wrong to make my fiance choose between me and his son?
Are you certain that this is what you're doing? No parents worth their salt will ever abandon their child for a love interest, especially not
if the child is suffering abuse. (I suspect you know this because you seem like an intelligent person.) So by telling your fiance to choose between yourself and his son, you are, in effect, breaking up with him. Perhaps you should just call it what it is and move on.