Welcome to the forum! I am sorry your betta isn't feeling well! We will need a little more info to help you though;
how big is your tank:
how often do you clean your tank:
how often do you feed your fish:
what do you feed your fish:
how often has your fish been showing these symptoms? Have you treated him with any medications?
have you changed anything in his tank recently?
have you tested your water parameters? If so:
also what do you add to your water with each water change?
how long have you had your betta?
sorry about all the questions, but we want to make sure we can give you the most accurate advice. As of now, i recommend a water change just to perk him up, as water changes are usually the best remedy for many illnesses or even just to perk up a fish.
you can also visit this website for further info:
Good Luck for now!