I'm a worrier, so if it were me I would take him to my regular vet for a good checkup. It could also be from a change of diet, or is he on the same food he was in the shelter? Since he had a URI they can come back so I really think a checkup would be a good thing! Also he should be checked for diabetes if he continues to drink a lot and hang out at the water bowl. Good luck and let us know!!