Will be good if you have picture for bettas fish too.
1. first time if you seen open wound recommend use sea salt for protect him from parasite and bacteria firs. (1 tea spoon per litter) just first time
2. incase for fins and some fin loss
Fin rot can be caused by two Bettas fighting but can also be caused by the water quality of the tank. The bacteria that can cause fin rot lays dormant in all tanks but becomes dangerous to your Bettas when the tank is dirty, over occupied, the fish becomes stressed or injured.
Torn fins, inflamed rays, blackening or redness along edge of fin or tail, blood on the tips of fins and receding fin edges. should be keep freshwater for him
3. I’m not sure that is big belly?
If big belly that can make sure is dropsy. You cam follow this article for cure dropsy.
Hope your fish back to healthy soon.