I know I'm new here and all that, but I really wanted to chime in real quickly. Apollo is a young boy, but I've had issues with his digestive track for months. I could go into the details and the medication and the tests, but that would take me an hour to type out.
ike others have said, I also think you're mixing his food too much. That can upset his tummy more and more. I would seriously consider taking dry kibble out of the picture completely and go with a good quality grain free canned food. PetSmart carries some flavors of Wellness, but they're pretty limited for options of good food.
If you find a small mom and pop type shop (or if you even work at one?), they should have a bigger selection of high quality grain-free canned. If the selection is limited, you can ask them to order more. I have to do this now, lol.
I give Apollo (keep in mind he is less than a year old,) Wellness and Wellness Core. Chicken, Turkey, Liver, and some fish flavors on occasion. I'd rather go fish-free because a lot of cats have weird allergies to fish, but .. I just limit the fish to once a week. He isn't a big fan of venison or beef or duck, even.. So it's difficult.
Anyway! You could try Evo, Wellness, Innova, By Nature, etc. Just keep in mind, that if you start feeding him canned-only, be certain to mix a different brand 50/50 with a brand he's used to. A lot of them have different amounts of proteins and etc that can actually upset kitty's tummy. It doesn't have to be a weeks-long transition as with switching dry foods, but it still can upset kitty's tummy.
If you want more options, you can google grain-free canned food and a lot of different brands will come up. Good luck!