Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your situation, and wanted to help.
Again I do apologize that my colleagues could not aid you sooner. If you would still like assistance, can you tell me:
How long has he been losing weight and vomiting more?
What food is he on just now?
Does he have any gagging, retching, drooling, or lip licking?
Does he look like its difficult or sore for him to eat?
What tests did his vet do? Just a stool test?
What was the medication they gave?