His symptoms sound consistent with liver shunt. Many vets aren't knowledgable when it comes to this disease. You can search here and find loads of information on liver shunt...or perhaps google. To rule this out...he needs to have a bile acid test done to determine how well his liver is functioning. The results of that will tell you whether more testing is required.
I'm sorry he is sick and hope you find a vet who can help him.
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