Please don't take it personally, but this is an appalling set up for a betta you have there. Do you realise that they need big surface area to breath? He can suffocate in this bowl!!!, especialy with that betta bed blocking the little available surface. He looks to have curling at the edges of the fins which I suspected on my own betta when I got him but yours look even more so. It can be caused by bad water conditions, small space, infection, etc...
Here is a video of my betta that I got a month ago who lives in a cycled, heated, planted 7G tank and although he'll never completely recover the damaged fins from the rough life he seems to have had before, he has become really active and happy.
I'd suggest get yourself a small 5G tank with heater and filter, read about how to cycle a new tank and provide you pet with a life he deserves.
Here is my betta that I saved from the pet shop. He was lying on the bottom of his tiny cup listless and look at him now in my tank: