Op, I agree with getting some gastric testing, if possible. I may or not be divergent, testing is cost prohibitive. But I was diagnosed as a child with IBS and still have occasional issues after food poisoning set backs a while ago. After getting that straightened out, a decent probiotic cap with fiber set me back on track until I could eat a mostly veggie diet, until the robber Barron corporate overlords started pricing decent food so crazily. You can add nutrition to crackers or bread with nut butters if your toddler will have them. If toddler requires more sweetness, try adding as little maple syrup as possible, and honey if his practitioner deems it ok.

Have you tried home made sweet potato fries (oven baked or air fry is fine, you'd have to look up how to oven fry them), or if your child will eat baked sweet potatoe with a little real butter they are highly nutritious and gut friendly. It's just hard to say because one of the many reasons I may be asd is it doesn't matter how much I like the flavor of something, if I don't like the feel or consistency of a food, I'm simply not having it.