Vita-Chem and Garlic Guard daily added to the water will be good for him (1-2 drops of each per gallon). Fish Protector is very good as well, but would need to be ordered online from a place like
www.thatpetplace.com.
The best food for him now would be Hikari frozen bloodworms. Get the package that has sheets in it, then you can break off a small piece and put it in a little container with treated tap water. Shake it up with the lid on and that's how we thaw the worms out. You can store them in that container for up to a week in the fridge and feed him 3 worms per meal, 2 meals a day.
Ocean Nutrition Betta Pro pellets are what we recommend as a secondary food, but our Bettas main food is bloodworms and that's even moreso when they're needing to heal up.
Once a week, I would feed him 3 pellet-sized pieces of a frozen pea, thawed and peeled. That should help keep his digestive system happy.