This is a first. It sounds like your boss can't be bothered to collect evidence to build a case against you, so he is asking you to keep a list of your own mistakes to build a case against yourself.
I guess you can't refuse to do what he asked, or you could be accused of insubordination, which is a firing offense. However, who is to say what constitutes a mistake? If you don't think you make any mistakes, the list is short or nonexistent. If you do make mistakes, just pick the most innocuous one and put it on your list.
Meanwhile, do be sure to find out what your legal rights are. I'm sure Crohn's is recognized as a disability, and you can't be fired for a disability. But check with an attorney who specializes in disability and employment law.