Bettafix and any kind of medication with "fix" at the end isn't a good medication to use with Bettas. The tea tree oil in it affects the labyrinth (organ used to breath air from surface) negatively.
Do a large water change and add some aquarium salt at 1tsp/gallon there aren't any scaleless fish like Cories living with him, 1tbs/5g if there are. Along with some stress coat and/or something like "general cure" to his tank and that should help.
Red usually is a sign of ammonia burn, are you sure that the ammonia reading was correct? Are his gills inflamed/red?