My pufferfish is sick?

He used to be a healthy little puffer, but now he always stays on the bottom of the tank, is always a dark color, doesn't eat, and is turning very very skinny due to lack of food. We still feed him and sometimes he makes an attempt to eat the food. He like swims right through it, but doesn't eat it. His swimming is in jerky movements. I think either his teeth are overgrown, or he has internal parasites. However, we fed him krill almost daily (the other days he had bloodworms) so his teeth should be grinded enough. I know what to give him to cure internal parasites, but I don't want to apply them until I'm absolutely sure he has that disease. Also, if teeth are overgrown, how am I supposed to cure his teeth? Do I file them, or what? He can't eat anything, so it's a too late to feed him hard stuff like snails. Please help! I don't want to lose my little guy. :(