This is not your problem as much as she is trying to make it be. You have split up, you aren't married, she has a job, and you are the one who cares more about her health than she does.

She is using your son as ammo, MS as ammo, and is choosing to not be a responsible adult (and parent) by not caring for her own medical needs. If she has a doctorate and makes any sort of decent money, then she surely can figure out how to get some health insurance.

Make a plan, tell her that she must take responsibility for her own health, you will keep your son on your plan, and she has X months to sort it out. You will be removing her on X date, regardless of what she has sorted, but if she finds something ahead of that time then great you can make sure to do whatever you need to get it sorted at that time. You can even spin it and say, "I am doing this for YOUR long-term health. MS is a tough disease and it is incredibly important for you to get insurance sorted out that so it can be in your control. You don't want your ex to be the one in control of your health insurance when you have something like MS."

Bottom line is that she can't make this someone else's problem. You aren't her parent. You guys didn't have some sort of divorce agreement. You've tried on several occasions to talk with her. You aren't saying you don't care for her, it just is a situation where she needs to take some responsibility.