Sudden emaciation is usually caused by internal tumor growth. In this case, your bird likely won't make it. Treating with Hypericum will stop the tumor growth, but once they've reached a certain point the damage cannot be undone. Give the bird two weeks of treatment, after that point, if you see no improvement, put him down, he's in a lot of pain. Crush his chicken feed into a powder and mix it with a little yogurt (thin enough that you can squeeze it through the tube). It's really easy to force feed. Just be sure not to shove it down the trachea (that's hard to do though...). Good luck!