He's pulled a serious joke on me in the past so I can't ask him. Though this would be going a bit too far. Anyway, I don't want to talk about it anymore.
It is a public forum, so you already have the information.
You have been talking to him very recently, so whatever the 'serious joke' was I can only assume it wasn't bad enough for you to discontinue your interaction with him.
You made your own views clear in an Underground thread here about another poster talking about some issues on a public forum, and yet here you are doing the exact same thing about someone's possible serious health issues.
You can say you don't want to talk about it anymore, but you are the one who posted about this person completely randomly. I think it's very sad that you wouldn't reach out to him given how far back you go.