Advanced Fin and Tail rot
•Symptoms: Fins and or tail start rotting away, usually starting from the edge, but sometimes it starts at the base of the fin (especially dorsal) and attacks the body directly. Diseases progresses rapidly as the tissues are being eaten away. Once fins have been consumed, rot will proceed onto the body. At this stage the disease is hard to reverse although the betta might continue to live for months if treated properly. If not treated, it will die promptly.
•Treatment: If the rot has advanced onto the body, skip conservative treatment. Do 100% daily water changes. Use a combination of Fungus Eliminator and Tetracycline. Continue until fins/tail stop receding and start showing some new growth. It may take up to 4 weeks to work, so don’t give up. http://www.bettafish.com/99-betta-fish-diseases-emergencies/73332-betta-fish-disease-treatment.html
I'm afraid to say but I think it's best to euthanize him when it's bad like this...