Are there any spots on his fins? Does anyone smoke in your house? What is the air quality in your home? Bettas are air breathers, they breathe at the surface so toxins in your home will affect him. Is he eating betta food? Do you have a thermometer to watch the water temp? When you change the water do you use tap water? Good luck! There are some good forums out there with fish experts to that may help if you google it.
I prefer my people to wear fur..
Gonzaga's Lady Perdita "Lady" CKC Yellow Lab
Adoptee Gucci super fuzzbutt
Adoptee Prada crazy dog extrodinaire
Tickles Tortie cat the owns my daughter