Your son sounds like my youngest - she was the one most codependent and most protective of her father.
What happened to us is that AXH eventually treated her the way he treated everyone else, and she had had enough and is now the least compassionate when it comes to her father.
I don't think for a second you need to be compassionate with your ex. You're divorced, which means what you choose to do with your life is none of his damn business.
I would, however, be concerned about how your choices affect your relationship with your son. I know that my kids have very mixed feelings about their stepdad - mostly because they feel he imposes on their time with me. I can imagine that adding two more kids - step siblings- to the mix could make that worse.
New husband and I also agreed that we would marry before moving in together. Our thinking was that we had both seen the kids react to our exes introducing them to scores of boyfriends/girlfriends and we didn't want them to go through that; we wanted them to k ow that "we" were for reAl, that it wasn't a fling and that we were serious.
I can imagine it's tempting to go ahead with the moving in for financial reasons. I'd be very very cautious though and I'm glad you are, too.