I am sorry about your son. It must be so tough.

You’ve been apart from your ex for 18 years so what he does or doesn’t do shouldn’t be very relevant. I’d say it’s understandable you feel annoyed dealing with your ex. I get along with my ex and he isn’t NPD at all but if I have to spend long time in his proximity (for our daughters events) I remember why we are divorced. He annoys the heck out of me.

Try to focus on your son and not on your ex. Kissing on the mouth is weird but perhaps it’s a non issue at the moment. If you believe it’s negligent or abusive then talk to your sons medical team, otherwise you should let it go

I am sorry about your sons illness