First, I'm so sorry to hear about your diagnosis and your upcoming surgeries. 

It is totally great that there's a healthy dynamic between the kids. It's also great that your skid seems to accurately read the BM's behavior and senses that it's inappropriate. While I can't say for sure because I can't predict the future, no good will come of you trying to write to the BM! She might even respond kindly (big maybe), but in my situation, the BM tries to create a victim - rescuer - aggressor dynamic between herself, me, and DH all the time. And the BM in your case is already showing signs of high conflict, like trying to poison the kid against you, creating fake social media accounts, trying to take the kid abroad. An email is NOT going to fundamentally change this person. Life IS short and precious, and as much as you can, I would stop giving your energy and compassion to this person because it's just going into a blackhole. You CAN be loving, accepting, and comforting to your skid when he's upset by her behavior (without saying bad things about her), but really and truly, you can't fix what you didn't break, and it sound like she needs more than an email to really get right.

You are doing the right thing for your SS.  Even at his young age, he understands and appreciates it.  He also sees that his mom is not acting correctly.  Keep doing what you are doing, don't badmouth BM to him or let him overhear you saying anything negative.  As a BM and SD of 5 who are all now in their 50's, I believe children see things with laser clarity, ours did, the good and the bad.

This BM is very high conflict.  Don't write to her or do anything else.  When you must be in her presence, be polite and civil.  Doing anything, reaching out, is a waste of your precious time and energy.

I am so sorry to hear of your illness and hoping for your best possible outcome.  Good luck.

I will reiterate what everyone else is saying. Dont waste your time. Focus on your bio, getting healthy and your marriage. SS is seeing how things go with his mother, and he knows what shes about.

MY particular brand of advice would be to make certain that SS knows that "he doesnt have to choose between you, he can love EVERYONE". I have the same situation, and Munchkin SD14 and I have a great relationship. Her mother is super toxic, especially to me, but also DH. Munchkin sees this always. But, again I make certain that she knows its ok to love everyone, there is no necessity to choose who to love and who not to love. This is important so as not to activate any loyalty binds in the child.

Of course stay positive about the mother. And support your husband. Talk to him at length about the need to create and enforce boundaries. If he didnt have them before and suddenly things change, yes, you will be called controlling and he will be called pathetic, but so what, as long as boundaries are created and enforced.

I just wanted to say that I've experienced so many of the same awful things that you write about here! While I'm absolutely in no position to give any advice because I have no clue how to deal with this stuff, I can say that the comments and suggestions given by others here seem wise (and I should take them into account too!).  I am just starting to come to terms with the fact that no matter how much I want to write BM, she has a) never responded well and indeed often made things worse after I've written her and b) she will never change and if she does it sure as hell won't be because of me! Hang in there, it must be absolutely horrible to have that toxic energy in your life, especially when dealing with so many other serious things. I wish you a speedy recovery!! 

Definitely all of the above. Focus on your home, you can say nice things about BM in SSs presence so he doesn't feel uncomfortable.

But do not waste anymore of your time, effort and energy on trying to have a relationship with BM. Because the bottom line is she doesn't want one with you. Any attempts of her being nice to you, is due to an alterior motive on her part and will not be genuine.

I made reasonable effort to be the good guy with BM in the beginning. The minute I realized she was a lying, conniving, manipulative child. I cut off all contact immediately. I don't even acknowledge her existence. 

From my experience, it's best to focus your energies on what you can control in your own life, which would be the relationship you have with your SS and how his presence impacts your home.

BM and I had a rough introduction. DH introduced us, she looked me up and down, and turned her back to me. Cool. It's not like we were in public, in front of her kids or anything (oh, wait, we were). I heard later that she talked crap about me (not surprised, she did the same to DH). We did have a year where we all got along(ish), but I think it was a similar situation to what you described. She was either trying to impress someone, or it was a good year or something.

We're not enemies now, but I have little to no use for BM in my life. Her kids see that she has been part of the problem, and it has bit her in the rear. I don't wish ill will upon her, and I truly hope she gets her sh*t together. But I have stopped trying to focus any energy on a relationship with her and focused it on my marriage and helping my DH raise good sons.

It's not easy. I still get angry with her. I still roll my eyes when I hear certain things. I make comments I should probably keep to myself (not to her or the kids, but to DH). But, I don't consider her feelings much when I do parental stuff. Sure, I'm not going to participate in a mother-son event, but I'm not going to worry that if I take pictures of the boys that it will upset her. Had she not wanted another woman in her kids' lives, she should have worked harder on her marriage to DH. I'm not going to feel guilty for existing and making connections within my own family structure and household.

All that said, your DH needs to create some boundaries with her if he hasn't already. I wouldn't advise that he speak up every time she opens her mouth. However, he should put a very clear boundary between your family and her. In your home, things run differently. In your home, BM is to be respected by her son (and you two in public - privately, it's different). In your home, your DH talks through things with his son and sheds light onto lies that BM may say. DH should be telling BM no and telling her when he hears lies about him so she knows that he's not in the dark, but it needs to be factual and focused, devoid of feelings. A "you told SS this thing, it's not true, I corrected it, and I will continue to correct him."

BM is your DH's battle, not yours. Focus on what it within your realm, and leave her in his. You can be supportive of him amd even offer advice if he asks (and he listens), but this is fot him to deal with. He, not you, produced SS with her. If he didn't want to deal with her, he shouldn't have had a child with her. Sounds cruel, but it's the truth. 

So stop trying to befriend her. Stop thinking you're going to change her mind. She's purposefully being obtuse, just like you're purposefully trying to be kind. It's a choice on her part, and you can't/won't be the one to convince her to be better.