There really isn't any "best food" for IBS...every cat diagnosed with it is likely responding to any one (or more) of a number of things and it varies widely. Some have problems with corn, but not other grains. Others can't do any grains at all. For some it's certain proteins. It could be a response to different additives like carrageenan. And sometimes it doesn't matter what food you feed, none of it works and the cat needs to be on immune suppressants (my Kobi was one of these cats). Dry food is often very hard for them to digest. Some can't eat processed foods and need to be on raw.
I suggest you look for a high quality canned food with no grains, by-products, artificial color, or soy. Stick to a single protein choice that is something he has never eaten before. Feed nothing but this food for at least 6 weeks. If symptoms clear up, then you can try adding things back in one at a time to see where the problems lies.
I don't know enough about the foods you mention to know if they're good choices and the websites don't provide much info. One of them says their food is 65% chicken and doesn't mention anything else but vitamins and minerals. I know those aren't the remaining 35%. So no idea what else is in there.
In addition to the Nature's Variety Instinct already mentioned, brands that may be available to you that would be good choices are:
Hound & Gatos
Natural Balance Limited Ingredient
Any brand that offers a 95% meat line
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Holly, Misty & their friend, Jake, the dog.
Onyx, Callie May, Maggie & Kobi forever in my heart.
Yes, I know I need a new signature pic...someday...