The sad fact is that we none of us have the control over situations such as you describe, nobutts, and all you can do is to mitigate as best you can, which is what you are doing.
Unfortunately, your DS is seeing first-hand, that adults do split up and people can disappear from our lives. What he will experience is the supportive relationships around him, people who love him, and this will become his reference point from which to become a strong and emotionally sound adult.
As a child, I experienced what they used to call "a broken home" and I never remember anything ever being explained to me, so it was awful, painful and confusing. Thankfully, adversity made me strong and tough (but thats only a 'front' 
. The big difference these days is the received wisdom that it can make a big difference if you give helpful explanations and support to the child according to how they feel at any given time (after all, it is all about "the immediate". at that age).
I agree, it is critical for your exH to put on a brave face, and to 'frame the situation' from DSs perspective and yes, you said it, suck it up. He is the adult, and thats what happens in adult relationships, but DS is still a child so the focus ought to be on his needs first. But, I recognise thats easy for me to say, I am sorry I dont want to be harsh to your ex's situation, it was a long term relationship. Hope things pan out OK for you all.