My brother used to be like that.... well the not gaining anything part. and my friend, no names, is like that... but she can eat. I have like 3 suggestions....
1. go to a different doctor for a second opinion
2. go to the hospital
3. try protein shakes, they replace a meal. maybe you wont get so full and u'll want to eat more as time goes by.
Keep us posted to let us know how ur doing!!! Best Wishes!!!