Will need a fair bit more info to be able to even hazard a guess at what's wrong.
Have you checked if he's eating normally, and pooping normally? Does his crop empty overnight? Dramatic weight loss can be from starvation, dehydration, cancer, viruses, infection, a lot of things really.
This thread I've linked to below has some questions that are necessary to answer, because without this info nobody can help, but unfortunately nobody seems to answer all the questions even when I list them and tell them why we need this info and ask them to quote it and put their answers beside it...
So I am going to recommend a vet, chances are they will get to the bottom of this problem quicker than us guessing at it.
But I know not everyone can afford a vet. If you can answer the questions in this thread below it may point to something, an ID on what's wrong, which can point to treatment options in turn.
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This is a link to a lot of info on various diseases, if you scroll down you will find a simple diagnostic chart to help figure out what's wrong by the symptoms shown. Some diseases have identical symptoms but this may still help:
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Good luck and best wishes.