if he has a wound or a hole it sounds like it could be a bacterial infection.
you can do a search using the search function on this forum or go to bettatalk.com and look under betta diseases to see if you can find something more specific. This is the advice I swiped from bettatalk about dealing with bacterial infections:
Do a full water change for jars or a 70% water change for tanks (to get rid of some of the bacteria present).
Clean filter, change filtering system, remove any uneaten food rotting, or any dead fish. Add AQUARIUM salt (1 tablespoon per 5 gallons).
Isolate the betta if he's in a community tank.
If the carbon in your filter is less then a week old, remove it otherwise it will absorb all the medication and you will be flushing your money down the tube.
Try Tetracyclin, or Marcacyn I and II combined, or Methylen Blue, or Maroxy or whatever you find at your store. Go for “broad spectrum” antibiotics, though. Follow manufacturer’s instructions and don’t stop the treatment until the betta is well again.
hope that helps
