I had terrible trouble with Djembe with the exact same symptoms! My vet worked so hard, making phone calls to exotic vets,etc. Djembe is an F4. I tried pumpkin, green beans, all kinds of homemade food ( this was over a long period of time). He even dripped bloody diarrhea when he didn't know it, like on chairs, tables, the floor. We FINALLY took him to Virginia Tech vet school. They put him on a Science Diet canned prescription diet ( I can't remember what, maybe A-D) and within a week....all was cured! Maybe he just outgrew his problem, but he's four and has never had the problem again.
Has he been checked for T foetus?