I love betta. He's depressed. Yes, fish can get depressed. First off, what size tank is he in, how are the water levels (ammonia, Nitrates, etc), and is he sick? Does he have ich or a fungus? Does he have spots or is his tail looking ragged? Is he bloated? Anything hanging off of him? Are his scales sticking out like a pine cone? If the answer is no to all of these, and nothing's physically wrong with him and he has a healthy environment that's atleast a 2 to 5 gallon tank with weekly water changes and delicious food, then he's depressed. I had one that was like this. He moped around, he just sat at the bottom, only came up for food. I moved him to a ten gallon tank, got a tank divider, and another betta. He then went from sad fish to I'm going to show this guy my moves and to an active and happy male betta. You can ONLY do this if you keep them separated though. If they actually can touch each other then there will be blood. But in the same tank (10 gal or more) two males still can establish territory and still meet up in the middle for a flashy showdown. (keep in mind the tank divider- a clear plastic one is best). If you do not want another betta, then a mirror works, but they tire of it quickly and eventually go back to moping.
But if you have him in a tiny tank, less than 2 gal, then he's really bummed. Get a bigger tank with aquarium plants at the very least, you'll be happier that you did.