It sounds like he has a bacterial infection of some type, unless you have him in an uncycled tank in which case he could be getting poisoned in his own
ammonia waste. Do you have a
test kit? We would be able to tell you more if you could send us the test on the water for ammonia, nitrites, and nitrates.
In the mean time, it would not hurt a thing to do a massive
water change, like 50% of the water with water treated with a good water conditioner like StressCoat, NovAqua+, Amquel, or Amquel+, or Prime (the last three should not be used in an uncycled tank). Then get a good antilbiotic combination like Maracyn and Maracyn2 and give according to directions on the box for the size of your tank.
What is the size of your tank? Is it heated and filtered? What temperature do you keep it at? (Bettas need to be kept at a steady 80 to 82 degrees F or 27 to 28 degrees C) What are you feeding and how much?
Please read the Welcome and
betta Care Guide in the topics at the beginning of the topics of the Betta Section. I know that there may be something that would help or give you an idea.
Please post any questions or comments and his progress. We are here to help if we can and you are also free to read all the posts to see if the experiences of others may be of help to you. We all love our fish and will be more than willing to welcome and do what we can for you. Comment, question, and enjoy, we are so glad to meet you. If your little one has a name please introduce him/her as here we love to call them by name as we believe they are all little personalities and as such deserve to be treated as themselves.
Let us know how things are going please and if you can the information we need.
Rose