It sounds like he has columnaris, which is a bacterial infection. I would treat him wiht a stronger antibiotic than bettafix. I know maracyn, kanacyn, eythromyicin, and tetracycline are all supposed to be good. Have you checked his water stats?
Also, add some aquarium salt into his tank, as that can help.
Here is some more info. I am not the writer of this.
Columnaris (a.k.a. Mouth Fungus)
Symptoms:
An infected fish will have off-white to gray cotton-like patches on the head, fins, gills, body and particularly the mouth. In time, these areas will develop into open sores. Gill swelling may occur, gill filaments may stick together and excessive mucus may develop in the gill area. Rapid breathing can be seen. Fins may deteriorate to the point of leaving the fin rays bare. Muscles may be inflamed and capillaries may rupture. Fish, particularly livebearers, may exhibit "shimmying". Infection may be acute (killing an infected fish within hours), or chronic (lingering for several days before eventually killing the fish). As with most diseases, not all symptoms need be present.
Cause:
The bacterium Flexibacter columnaris.
Treatment:
Ensure that your water conditions (e.g., Ammonia, Nitrite, pH, Nitrate levels, and water temp.) are within their proper ranges. If not, perform a water change and/or treat the water accordingly. Recommended medications include: Furanace, Fungus Eliminator, Fungus Cure, Furacyn, Furan-2, Triple Sulfa, E.M. Tablets, Tetracycline, or Potassium Permanganate. Medicated foods are also recommended. Columnaris can be highly infectious and may quickly kill all aquarium inhabitants; therefore, early treatment is essential. All fish, including those not yet showing visible symptoms, as well as the aquarium they inhabit should be treated.
