Hi A. :)
Based on what you've written, I can completely understand how you would be so frustrated and at the end of your rope.
Have you tried going to counseling? Either for yourself or with the two of you? Sometimes it's really helpful to have a moderator on board to help each of you hear and understand where the other one is coming from.
I know it's hard to bring someone else into it, especially when "family business is kept in the family". Of course, years ago when your parents were growing up, that probably made sense because they probably had family or had parents themselves who had family that served as counsel during times like this. But nowadays we don't rely on our parents, grandparents, aunts, whoever as much to see us through the tough times, and in the end the only people who lose are ourselves. You don't have to do this alone - it's ok to ask for help. There are extremely high stats of divorce for couples who have been through situations like yours, often because they won't talk to each other. Not talking anything fancy either - could be a free service such as a pastor or work program or other family service.
I'm a big believer in exhausting all resources before ending a commitment, especially one as big as marriage. Yes, sometimes divorce is the best thing you can do for your family. But with your son being so ill, it would be so difficult for you to grieve both the loss of your marriage and your son. (Yes, even women with the worst marriages/relationships in the world find themselves grieving when they're over!) So, for yourself and your son and your daughter (who is already going to go through a lot just by watching it all unfold), my suggestion is to find whatever means you can to communicate the urgency of the situation to your husband. I'm sure he's a good man at heart - there was something about him that made you believe he was worth marrying. This is got to be taking a toll on him too, and some men (people, really), are just paralyzed by intense situations and need step-by-step guidance to get through it.
My very best to you... (((((((HUGS)))))) this must be so difficult.. .
-R. Z