Betta Fish: Guys help what something happened to his mouth should I do?

your fish is rotting while still alive, if that’s his permanent tank then you don’t have enough water to handle the bioload, you don’t have a filter or any semblance of a beneficial bacteria colony to handle the waste, your fish is constantly swimming in water absolutely saturated with shit so it’s literally decaying while alive, prolonged exposure to waste in combinaton with a low temperature induced weakend immune system and constant chemical burns lead to tissue necrosis, couple that with the coctail of pathogens and parasites that breed in unmoving water and you get constantly open, infected, festering wounds, which is what is happening to your fish