Suggestion for FBO sufferers to isolate the cause
DISCLAIMER: I am not a doctor or medical professional of any kind, and nothing here is medical advice.
My quick suggestion is to try wearing a pair of Depends (or other brands) incontinence underwear. If there is any noticeable reduction in the smell at all, then you have likely established that actual fecal material in your colon/rectum is the issue. This will dramatically alter the course of treatment you attempt, as opposed to if the smell is coming out through your pores or sweat. Trying enemas to see if clearing out the fecal material gets rid of (or at least improves) the smell is another idea, a user here is doing that.
EDIT: I am suggesting trying an enema to try and see if fecal material in the colon is the root cause. I am not suggesting you start using daily enemas to control this, as that has all sorts of potential negative consequences. You have to very carefully approach the idea of doing that.