Bettafix is a watered down version of mellafix - both contain tea tree oil, which can harm a betta's labyrnth organ. Some bettas are very very sensitive to it, while others are not. However, it does work, but I'd recommend other options first.Remember - it MIGHT say "betta" on it, but it doesn't always mean it's the best stuff on the market.
It looks like a "Scrape", if he happened to have dashed around bumping into something with a sharp edge. Is there anything in your tank, like a hard ornament? Or another fish?
Aqarium salt, like SwimmR said. if you can, float him in a 1 gallon container, in his tank to keep the temperature warm. Dissolve 1 teaspoon of salt, in tank water, and add. I suggest no more than 10 days, as this will cause more trouble then good. 100% water change each day while on treatment.