Yup as Feng said, some Betta's turn colors through most of their life and sometimes it can happen pretty quickly. But a clearer picture will definitely help.
Also getting him so good food will help as well, New Life Spectrum Betta Pellets and Omega One Betta Pellets are great. Both have all the nutrients that they need. Also you can feed him a little more than two pellets at each feeding. I feed mine once a day usually, except Saturday they get two feedings before fasting, and I usually give each about 6 or when I see their tummies JUST starting to get full, not bloated or anything, but you can see them just slightly getting round. And then they don't get fed for another 24 hours or until they slimmed down again and poo.
Also I would stop the Aquarium Salt until we know exactly what's going on, just don't want to stress him out any further :)