I agree with 1f2f and CR-that is a male with terrible fin rot. Fin rot CAN and does lead to a more advanced stage-body rot. If you don't do something soon, your fish is going to die. It's not fit for breeding and you will only kill it.
I also agree with FloridaBettas-if you can't tell your betta's gender your shouldn't be breeding. If you know "enough" (IMO) about breeding, which means lots and lots of research, you should be able to tell if your betta is a male or female.
Also adding aquarium salt (NOT table salt or salt for saltwater tanks, but freshwater aquarium salt) will help in addition to what 1f2f said.