What's his poo like? If it's like string and has a lot of white in it, he could have internal parasites.
Normal betta poop is the colour of whatever you're feeding them. If it looks pretty normal, slowly increase his nutrition with live food and good quality pellets.
I've had two very thin fish - one had a terrible parasite problem that led to many other health issues until he died. The other is just weedy and stunted, and never has grown or filled out, he also has many health issues but is still kicking on -- they are from the same store, my guess is the same supplier, too. All my other fish are fat and super healthy. I've found good food and very clean water works to keep my weedy guy as good as he gets.