If he is badly burned he may not make it. Keep the water very clean, frequent water changes. With other fish you can add kosher salt to the tank to help heal but I am not sure about Bettas. You can get some tea bags and add them to the water. The tea will sooth the burns and promote healing. Add a couple of tea bags to stain the water or make a strong cup of tea and add it to the tank. You can also get medicine with Aloe and slime coat replacer at Walmart. If you have an Aloe plant you can add some of the gel from the leaves directly to the tank and maybe put some on the fish. Soak his food in some garlic water and see if you can coax him to eat it. My Betta attacked his heat and got a small burn when I first got him. It healed in about a week. Good luck.