You should feed him frozen brine shrimp. I used to feed mine hikari bio-gold pellets but i noticed mine never pooped. But after feeding him the shrimp a couple days he poops 1 time every 2 days. still trying to get him to 1x a day. He LOVES the brine shrimp.
My betta has white patches along his body. Some experts on here said it was body rot. I've been treating him in a hospital tank for about 2 weeks now and they havent changed much. But he hasnt gotten worse either. Its a long road to recovery. Check out my post for pictures to see if its similar. its called "cloudy white patches, puffy eyes". Its got a lot of info in it.
You should try to get a better picture so we can see more detail. Shine a lamp on him to take it.
Also what are your water parameters?