Fenbendazole will treat the cecal worm, but will not treat the protozoa that causes it. It can be treated, but the drug is banned for use in food animals, so one would have to consider that. The drug I use to treat blackhead is metronidazole, which you can buy online as Fish-Zole or in Petsmart it's called API General Cure.
It's also possible that it's coccidiosis. Has he lost weight?