First off, you have a beautiful family, . Secondly, I am so sorry you do not have a support system.
As a child of divorce myself, I have had a front row seat to good and bad co-parenting (and everything in between). I don't know how good your relationship with your ex is, but can you frame this as helping the children? As in, his willingness to help more with the kids benefits the kids more than anyone else. And, as an aside, the kids may read his resentment as having to do with them and I doubt he wants them to have that impression. I mean, I wouldn't tell him that, but kids do tend to put themselves at the center of things and may assume that their dad doesn't want to be with them or help them if he acts angry or resentful when you need more help from him.
Is family therapy an option? Goodness knows, as a single mom, I know your time is limited and precious. And trying to find good mental health care since 2020 has been, well, a challenge, shall we say? But, I am throwing it out there. There are therapists who will allow you to pay on a sliding scale and online therapy is an option now, too. I just know that having a neutral third party can sometimes help iron out details and help couples/parents make healthy, productive changes in a timely fashion. And, a good therapist can help keep discussions from devolving into unproductive arguments all the time.
Can you share articles or information with him about MS? Would he be open to that? Also, I wanted to share some tips from another MS community on parenting (but not necessarily co-parenting) with MS -- https://www.mymsteam.com/resources/parenting-with-ms-tips-for-day-to-day-challenges. Also, the NMSS has some good resources and support (both in person and via phone/online) that may be helpful for you. Here's one link -- https://www.nationalmssociety.org/Living-Well-With-MS/Relationships/Parenting. I personally have recommended their MSNavigator program for years -- https://www.nationalmssociety.org/Resources-Support/Find-Support/Ask-an-MS-Navigator. Here's a link to a great PDF that helps explain MS to kids (but frankly, I found it super helpful for adults, too) -- https://nms2cdn.azureedge.net/cmssite/nationalmssociety/media/msnationalfiles/brochures/brochure-someone-you-know-has-ms_-a-book-for-families.pdf. You may notice I am providing a lot of information for you, but that's because neither of us can control what your ex husband will do or how he will respond to your suggestions. But, what I can do is provide resources for you to be in the best possible position (mentally) as you navigate this new life your didn't ask for. Being a single mom is HARD. Living with MS is HARD. And I know MS is probably the last thing you needed right now.
But. You are tough. You are strong. You can do this,, even when you feel like you can't. But, getting your ex on board would definitely make things easier on you and your kids. Maybe we can find a way to explain things without making him feel defensive? Like, "I don't want you to feel like I am taking advantage of you, but the kids like spending more time with you and having you available to help as I adjust to this diagnosis and treatment will be such a gift for the kids and I. Your presence and ability to pick up the slack will help them feel some sense of normalcy in this very abnormal time. I appreciate your help and I am thankful I can call on you when necessary." Ok, I don't know your relationship, so that type of comment may not work or may need to be changed a lot to suit your relationship. It's just an example.
Apologies for the novel, here, but I want you to know I see you. I see how hard this is and I want you to know that. You didn't ask for this fight, but you can do this.
Also, as you get a handle on your MS and find a treatment that works for you (and learn any triggers you may have and how to avoid or manage them), things should get better. You guys will adjust and things won't always seem quite so overwhelming and bad. You have a whole community that has weathered the storm of a new diagnosis and many of them are parents, too. So, they get it. They may not get 'all of it', but they get it. You are not alone here.
Best, Erin, MultipleSclerosis.net Team Member.