Depending on how bad his condition is and what ailment he might be suffering from it might be best to put him down. If it is just fin rot or a small fungal infection you can easily treat him and nurse him back to health. Bettas are very resilient and can easily bounce back.
He could be suffering from Internal parasites, try soaking the food you give him in garlic. Most fish will go for it. If he does eat it, use medications for internal parasites (Prazi Pro) to treat him. Keep his water clean and keep the temperature and water parameters consistent.
If you decide to euthanize him, please do not flush him or freeze him. Clove Oil seems to be the most widely adopted humane method of doing it.
He could be suffering from Internal parasites, try soaking the food you give him in garlic. Most fish will go for it. If he does eat it, use medications for internal parasites (Prazi Pro) to treat him. Keep his water clean and keep the temperature and water parameters consistent.
If you decide to euthanize him, please do not flush him or freeze him. Clove Oil seems to be the most widely adopted humane method of doing it.