Columbo was just diagnosed with a protein losing enteropathy caused by severe IBD. He had no symptoms until he had a pleural effusion and severe hypoalbuminemia. He has been put on Purina EN and so far is doing OK on it, although he is on steroids and metrazonidole also. Here is a good article my husband found and it talks about food. Maybe it will be of some help to you. A lot of your pup's symptoms are similar to a protein losing enteropathy (PLE) or a protein losing neuropathy (PLN).
Protein-losing enteropathies (PLE)-2008 « Hungarovet