I totally understand where you're coming from!
I was going through the same thing ...and have been going through it for about 1 year now. I switched from nutro natural choice (dry) to innova EVO dry (along with some canned food) and kept my cats on it for about 6 months, I also tried taste of the wild (which they loved)...then one of them developed hair loss (possible allergies) and, more recently, one of the kittens developed an ongoing UTI...So, I've had to switch to Nutro Max (more UTI-friendly food).
The high ash, magnesium, (and something else I'm forgetting) levels in the grain free dry foods are UTI un-friendly, so not good for a cat prone to them.
Also, really check the ingredients on the "grain frees" I know that the solid gold grain free (blue moon?) has lots of potatos in it (just replacing one carb with another). Blue Buffalo's grain free is also like that.
Here's how my cats reacted to them:
Wellness core- tried a smaple size and now use it as a supplement to the nutro max. They eat it ok, but it is crazy expensive.
EVO-were on it as the sole dry food for a while... Good results, ate far less than on a normal food, less poo (and more compact), did not really have bad digestive issues, but they did have more gas on it and poo did smell more. It is cheaper than CORE.
Taste of the Wild- Cats LOVED it. they ate a 5lb bag. Did not have gas or stinky poo (like on EVO). Also it is WAY CHEAPER than the other grain frees out there. IF my cat could could back on a regular dry food...this would be the one I'd feed.
Also, IMO, these foods are more dense, calorie speaking (and protein and fat), so yes, I think they are rougher on the kidneys. I did note that my cats drank more water when eating EVO. I gave them a fountain to encourage it. I would advise trying to feed a good bit of wet along with the dry if you're feeding a grain free dry.
There are grain free wet foods out there...if you can get it locally, you might want to try EVO's "95% meat" line of cans. I'm not sure what it would be about wet food that messes your boy's stomache up (does he gobble it really fast? ), but maybe a grain free wet would be better tolerated....What I'm having to do now (due to the UTI issues) is feed the nutro max dry, but I'm feeding about 50% wet food...most of which is grain free...(EVO and By Nature) along with some natural balance.