I am sure there are yt members who know more about this then I do... but I can tell you what I have done.
I would mix a little bit of the egg white in his kibble... or just pour a bit of the tuna water over the kibble. I have read Parmesan cheese-- just a little -- grated is ok. Of course I would clear this with your vet.
I also would say get the best kibble you can afford. I feed my babies FROMM
now. If you go to a pet store that specialized healthier foods they could recommend to you Read the labels just like humane food