Now, at this moment, when everything feels topsy-turvy, don't let it turn into that old joke of a wife walking in on her husband in bed having sex with another woman and she said, are you having an affair? And he says "Are you gonna believe me or your lying eyes."
He's lying...you wouldn't be posting if you didn't believe it.
Put the responsibility square on his shoulders.
Tell him that he needs to go get tested for STDs since he might have caught something that could infect the kids. Hepatitis for example.
Don't let him wiggle. Just say, based on this letter, we need to protect the kids, and you can make them sick. And make sure to add that he needs to get the kids tested too since he might have accidentally passed it on to them.
Hepatitis can be transferred through body fluid, so if they drank after him, used the same silverware, or he kids them on the head even.
