Have you had his blood drawn lately? Just for a baseline? If his kidney values are normal, (BUN and Creatinine), then I would have no worries about feeding a high protein food.
From Doctors Foster and Smith ... (someone asked the same question so I copied and pasted their answer)...
Q. Are high protein diets harmful to my dog's kidneys?
A. A rumor has been going around that high protein diets cause kidney disease. This rumor is false. High protein pet foods are NOT harmful to a normal animal's kidneys. As an animal's body digests and metabolizes protein, nitrogen is released as a by-product. The excess nitrogen is excreted by the kidneys. A high protein diet produces more nitrogen by-products and the kidneys simply excrete the nitrogen in the urine. While you may think this would 'overwork' the kidneys and lead to possible kidney damage, this is not true. The kidney's filtering capabilities are so great that even one kidney is sufficient to sustain a normal life. There are many pets - and humans - living perfectly healthy lives with just one kidney.
The myth that high protein diets are harmful to kidneys probably started because, in the past, patients with kidney disease were commonly placed on low protein (and thus low nitrogen) diets. Now, we often put them on a diet that is not necessarily very low in protein, but contains protein that is more digestible so there are fewer nitrogen by-products. These diet changes are made merely because damaged kidneys may not be able to handle the excess nitrogen efficiently. In pets with existing kidney problems, nitrogen can become too high in the bloodstream, which can harm other tissues.
Unless your veterinarian has told you your pet has a kidney problem and it is severe enough to adjust the protein intake, you can feed your pet a high protein diet without worrying about 'damaging' or 'stressing' your pet's kidneys. Also, you are not 'saving' your pet's kidneys by feeding a low protein diet.
"Having heard all of this, you may choose to look the other way.....
but you can never say again that you did not know." William Wilberforce 1759-1833