You need a heater. Betta are tropical fish and need a minimum of 76 degrees to stay healthy. I'm guessing this is why his fins or clamped, because he is too cold. So unless you keep your room at 78 or higher, you need a heater. You can get small heaters for small tanks. You will need a constant temperature between 76 and 82 (not always changing, pick a temperature between those 2 and keep it there).
As far as the lighting goes is there a light with his tank? If you only turn his tank light on and leave the room lights off, he will see his reflection. The same will sometimes be true if you just turn the room lights on and leave his tank light on. So if you have a tank light, only turn it on when the room lights are on, or he will see his reflection and it will stress him out.