Take what I say with a grain of salt, I'm a newbie with fish but have read a ton of junk on the internet.
If the coral is real in your tank (but dead), stuff I have read says it will raise the PH and the hardness in your freshwater tank eventually. Other then that, I have no idea on his eating problem. He is a very pretty betta :) I hope someone here can help you, help him get better asap!
I think you can google "dead coral in a freshwater tank"
Added in edit:
Above link is something I came across here. but like I said I am not sure it has anything to do with your problem :(