I acknowledge how immensely difficult it must be to be in your shoes and just want to make a small comment.

You wrote that reminding your adult son of his penicillin allergy is an example of trying to have an adult conversation. I think it is rather treating him like a child: an adult is supposed to know of his allergies and if he forgets, by now it is his problem and you shouldn't bring it up. You did all you could in the past to raise his awareness of his allergy I am sure; now, let him deal with it. If he gets a bad reaction, he will learn. He may be somebody who only learns this way.