I second the suggestion to order online if you can- I have my cats on Taste of the Wild- it's the cheapest grain free option that I've been able to find. I bet if you emailed the company, they'd be willing to send you a sample so you can see if your kitten will eat it.
I'd definitely do the worm testing if I were you. Though, my cat was very gassy as a kitten. At the shelter, I picked him up, he started purring, and then farted
. He did it several times on the way home too, and for the next few months. He grew out of it, though, and never does it anymore, so I think maybe their digestive tracts just need to mature a bit. Baby humans tend to have troubles with gas themselves, lol.