A couple of thoughts:
You said your son was mad at you that his father didn't come.
That happened to me with my kids, too. I felt like I had
taught them to see their father as a child, as a helpless hapless person who couldn't handle things on his own.
That was probably partially true.
But I also think kids see that.
And they see you (and me) as the only solid anchor point in their lives.
So they turn to you (and me) and want us to
fix things.
Because they already know (subconsciously) that the A parent won't. Doesn't have the capacity.
In retrospect, it was probably a good thing he didn't show. And seeing our children hurt by their A parent is awful. But it's reality and they do need to face it, at some point.