He looks kind of bloated. This could be him being fed too much. It's pretty easy to overfeed a betta. I'd say do a fast day (no food) and then feed him then next day very little. if you are using floating betta pellets only feed him a couple or if flakes just a few to make sure he's getting food. Then I might repeat as needed until he gets a little bit under control.
His fins do look they could be suffering from fin rot. There could many causes of this but most often it is due to dirty water/bad environment. It could also be caused by stress which leads to a weakened immune system.
How big is your tank?
how big is your water change?
Is there a heater?
what is the temperature of the tank?
Is there a filter
Are you adding water conditioner everytime?
Are you actually gravel siphoning along with your water change or disturbing the substrate to get any trapped detritus out with the old water?
what kind of chemicals are you adding to the water?
I recommend Tetra Lifeguard tablets. it is a really good basic medication that treats multiple things. I use them whenever add new fish or my fish get sick. I have used it multiple times with new bettas that I got that were suffering from fin rot and nipped/ripped fins and i saw visible improvement within five to seven days.
