Bettas tailbite because they are stressed, bored, mistake their long finnage for another betta, feel weighed down by their tail, etc. Halfmoons are especially prone to it. It takes some experimentation to figure out what works and doesn't work. Sometimes, smaller spaces help. Sometimes, larger spaces help. More decor, less decor, more stimulation, less stimulation... It's all about trial and error. Every betta is different. Just keep him in warm, clean water to prevent infection and feed him quality food and his tail should grow back. You can add some API Stress Coat too.