Betta Fish: is he dying?

He isn't moving as much anymore, the most he does is swim up to the feeding ring when I get close. That's also a new development, he never did that before so I assumed I wasn't feeding him enough even tho I alwaus give him the same amount so today I watched him for a few minutes to see how much he eats, turns out he didnt eat anything it all just dropped to the bottom of the tank, he's just there. The tank is fine, 50l/13gallons, I can't pinpoint any issue so im kind of at a loss rn. I also cant take a better photo when he's like this so I hope this is enough.