If he isn't divorcing her it isn't because of money. He can leave and be part of your family whether he has paperwork or not. Not only is he there but the fact that he hasn't followed through with the divorce is evidence enough that he isn't ready. He may be stuck emotionally in this relationship with her but in reality it doesn't matter. The bottom line is that he has to make a choice. Sometimes relationships fail simply because of timing. That may be the case in this situation. He feels stuck but he isn't ready to move on either.
For your relationship to work he has to really want to make a choice. He has to know what he wants and he has to see it through. You can't do that for him. If you feel that the ex is so intrusive that you feel compared to her then that is a significant issue. You may not be able to put her out of your mind enough to enjoy the relationship. This can be resolved but he has to want to resolve it.
It sounds like your decision has been made by his inability to follow through. Maybe it's time to let him go and realize that he just isn't ready to be happy with someone other than his ex.
Please press accept; this is the only way I am compensated