Cat does not want to transition to grain-free -- ideas?

I've been trying to transition my 6 month old kitten into a grain-free diet. His stomach could not tolerate Blue Wilderness Kitten, so I've been trying to move him over to Natural Balance Green Pea & Duck. He won't eat the Green Pea & Duck. Literally, he will not touch it. He would much rather eat the Iams kitten food, which is a) not grain-free, and b) likely not as good for him. Is it okay for me to simply slowly transition him completley to the Iams, or should I really keep trying with the Green Pea & Duck? (He will eat the Iams out of the mix of it and the Natural Balance ... and leave every piece of the Natural Balance in the bowl.) His stomach is somewhat sensitive, which is why I was trying the Natural Balance, but the Iams doesn't seem to be upsetting it either. Just trying to figure out what's the best thing to do for him!

From my experience, cats either like Natural Balance or they don't. And some cats will starve theirselves before eating something they do not like.

If your kitten likes IAMS, why not try to find a grain-free food with similar proteins. I know IAMS uses chicken; but, don't know what other proteins may be in the food.

You could try Merrick or PetSmart's brand of Simply Nourish. Some of the Simply Nourish is grain-free and others use rice or oatmeal in the formula, but no corn, wheat or soy, which I think might be an ok compromise.