How often do you change the water??? If your not changing the water enough, it could be something other than constipation.
With fasting, you shouldn't have these problems. However many people seem to say there bettas get more constipated on flakes. Try Hiraki Betta Bio Gold (I'm such a bad speller... but this is close). My fish love them. They come in a couple sizes. I get the baby pellets and feed 5 or so twice a day. That seems to be about right when I compare size to other pellets. If you get a regular size pellet, 3 twice a day.
Eight flakes might be to much, over feeding can cause these problems too. I'm not sure what size flake your using, but I just think eight flakes of any size would be to much. Why don't you try cutting it down to half that. Give him four or five each day and see if he improves. But personally I'd switch to pellets.
Also the picture doesn't really help me alot. The part of his body that would be... constipated is to blurry. Remember, their exit tube is located in front of their long stomach fin. I think it looks a bit swollen though, but from the picture it's really hard to tell.