Very beautiful and plumpy guy!
I have nothing against feeding a little more than normal, I keep my bettas on the thicker side but you must change the water more frequently than normal and with the size of your bowl you should be doing water changes everyday. From the look of your water I can tell it's been at least a few days since you last changed it.
Also, bettas trive in warm water. It will help his immune system and get his more energized. I suggest looking into possibly getting him a heater... he will love you for it.
I just love his coloring!