He may be sensitive to an ingredient. My guy battled loose stools for most of his first year, until he was put on a raw diet. He wont eat raw anymore since he got sick (really bad giardia), so now he is eating Acana Regionals and his stools are as good as they were with raw.
Was the powder Tylosin? How long did you give it for? I had to use that for almost 2 months to clear up whatever my guy had, along with metronidazole. I know how you feel though, because i've had the EPI testing done, have done so many stool samples, tried so many foods..Which is why I advise you really look toward the diet. Yeast can even cause loose stools too. Does he get probiotics? Have you tried canned pumpkin?
What foods has he been on? Have you tried grain free? Have you tried different protein sources? I dont know proportions for the chicken and rice, I just usually eyeball it.
EDIT: with the skin issue, it really sounds similar to my dogs symptoms when he had a yeast overgrowth. Antibiotics kill off all the good and bad bacteria, allowing the normal amount of yeast normally in the GI tract to take over.
Berlin v Hokschhaus HIC CGC TC