Just my two cents, I don’t think there’s a way that you can word the email that wouldn’t make the recipient uncomfortable. Plus, if they’re hearing both parents accuse the other one of being the problem, it’s really hard to suss out the truth from where they’re sitting and it might make them reluctant to invite your son to things.
I also think it’s pretty normal for not all parents to make it to all activities. They might not even notice and then you’d be drawing even more attention to it.
Our philosophy on this sort of thing is that SO won’t go out of his way to tell people that BM did something wrong. But he also won’t hide it for her, like if he needs to tell someone “I need you to do X because BM didn’t do y,” he’ll say it even though she doesn’t look great.
It might be best if you let it go in this particular activity? I will say though, you don’t want him controlling the narrative with, say, teachers or doctors - because that could have more of an effect on your son (ie teachers only communicate with him, then you aren’t getting and can’t act on important school info). So I suppose if this is something integral to your son’s life my advice might change.