Does he have known kidney disease? If not, there is not evidence that even very high protein intake causes kidney issues.
Generally speaking, turning yellow would be more of a liver issue anyway. Are the whites of his eyes turning yellow? Then that is jaundice and should be evaluated by a physician immediately.
Some people can take on a bit of an orangish tinge if they are eating a LOT of beta-carotene. Does he snack a lot on carrots?