Is your betta having any white stringy poo?
Could he probably be constipated? Its very hard to tell from the pic
To boost his immune system:
You can add Garlic Guard or garlic juice from minced garlic onto the fishes food. This will help to boost the fishes immune system, too, garlic acts as a food enhancer.
Vita Chem is a good source for additional vitamins. You can add it to the food and directly into the tank itself. Follow the directions on the bottle. This will help the fishes overall health