Update:
urine culture normal, xrays negative... With the exception of "slightly cloudy lungs" which the vet says may be the cause of his cough. She mentioned allergic bronchitis as a possibility. Everything else was normal, he even gained 6 pounds.... The plan: recheck weight in 2 weeks, and if he is maintaining, then we won't adjust the food intake. If he gains, we will adjust down a bit, but if he loses, then further testing will be needed.