Do a 50% water change, if you haven't already. This will help dilute any excess ammonia, nitrates, and nitrites.
To test your water parameters, you'll need to get the API master test kit, the liquid kind. Follow the directions and post the results here. You need to test the pH, ammonia, nitrites, and nitrates.
If the ammonia and nitrites are anything above 0, do a partial water change. If the nitrates are above 40, do a partial water change.
To help your betta heal, you'll need to do frequent small water changes to keep your water conditions perfect. You can dose the water with melafix (at 1/5 the stated dose) or with Betta Fix (same stuff, already watered down).
You should be doing weekly partial water changes, or more often.
What size tank is he in? Do you have a filter? Heater? What is your water temperature?