You mentioned maybe there were too many proteins in his diet.
It reminded me of when I got my six budgies. I started reading everything trying to do it right. Many people where talking about feeding sprouted seeds and many vegetables or adding protein.
Then in my research I found results that a bird had died because it's diet was solely sprouted seeds. I believe they said aflatoxins was the cause.
I found there were even dangers on their veggies.
I wonder if the culprit is not something on or in the food. Although like I said urine results are mysterious to me. I know nothing about them.