Hmmmm.... well there are several tricks you can try with the kidney. Remember he just needs a couple bites a week, so don't stress too much. Organs are only 10% of the diet and half of that is liver.
You could dehydrate it in the oven until its crunchy.
Sear in a hot pan with a bit of butter and garlic.
Freeze and try it that way, some dogs don't like the texture.
Squish it up or dice it and then mix with hamburger and make a little meatball.
Liquify in a blender and then mix with something he likes.
Dice it tiny and mix with canned food.
Smear with parmesan cheese or peanut butter.
Slice into paper thin pieces and hide it under the skin of a cornish hen chunk.
There's lots of little tricks to get them to eat their organs. If worse comes to worse, you can just pill them with it. Not my favorite but I know some raw fed dogs who absolutely just will NOT eat it, so their owners pill them with their organs once a week.
"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