I am in no position to help, as I am a betta beginner, but I do know what the people here who can helpd need from you. They need to know what his water's ammonia, pH, nitrate and nitrite levels are. Do you own a water testing kit? They sell kits with tubes and fluids (like a swimming pool test kit but specifically for aquariums) and strips. The strips aren't as accurate as the kits (and the kits are less expensive in the long run).
Until people know if your fish is in water with ammonia, nitrites, etc., it is difficult to determine the problem and help.
How often do you usually feed him and what type(s) of food?
How often do you change his water? How much water is he in? Is there a filter at all? (From the pictures, it doesn't seem so, but you should let everyone know so they are sure.)
I wish you and your betta the best!