Thanks for the note, and I’m sorry that the joke offends you and others. I’m afraid I am not going to change it.
"I'm afraid I am not going to change it"? How pathetic. You can change it or not, but if you decide not to, don't try to hide behind that sort of placating passive language. I'd have much more respect for him if he said "I'm not going to change my joke because I think CFS is a funny disease. It makes me laugh that people use it as an excuse to avoid getting on with their lives."
"I'm sorry that the joke offends you" - that's not the problem! It doesn't matter whether anyone was offended or not, the problem is that his joke was based on a misunderstanding of CFS, and may serve to spread that stupidity amongst his readers. He doesn't have the right to appologise for the justified offence which others may feel.