The task, as a parent, is to fashion a version of the truth that is age-appropriate. To translate what happened down to a level that will allow your son to know a version of the truth, and then allow the details and understanding of that truth to develop as he gets older and can tolerate more.
One of the things we have to be very aware of when thinking about how to make what we say age-appropriate is the natural inclination for children, in simple terms, to blame themselves for the failings of their parents.
While that may sound like a cliche, it's a reality that I'm confronted with most days as my clients wrestle with and untangle the events of their childhoods. It's almost impossible to get your mind back into the headspace of being a child, where our parents are our whole world - and that focus is needed in large part for survival reasons. But the flip side is the tendency for children to look to themselves to try to understand or explain their parents' failings.
In simple terms, if I didn't feel loved, it was because I wasn't loveable. If they left, I'm not worth sticking around for.
So whatever we say, and however we say it, it needs to be tempered with lots of explaining that your son is not in any way responsible for what happened.
By way of reassurance, the research is clear that talking about suicide to children in a clear and straightforward manner, while upsetting, will not increase the risk of suicide for him, now or in later life. It may, however, mean that he continues to grieve, as the loss of a parent does sit with us for life, as part of our story that can define and shape us. For good as well as ill.
This doesn't mean we don't "get over it". It means that grief comes and goes, and that piece of the puzzle will always be missing in some form for your son. However, grief also needs the truth.
So stick to the truth, answer questions matter of factly, stay away from talking about the specifics of "how" they did it, but give enough information so that he can understand. But also hold the memory of your ex compassionately, even if - understandably - you don't feel that way about them.
Because he was in pain, he was suffering from a mental illness that tragically killed him, and ultimately it was nobody's fault.