In my experience it's just the day that you eat it that's a problem. Ali said up to about 12hours for her. (J got hold of some apple yesterday when we were at our music group = 40min screaming session 2.5hrs later during nap time (woke having tummy cramps and just screamed). But when it was over he stopped screaming, calmed down pretty easily and then I took him back to bed and he was asleep again straight away. He did have diahrroea later in the afternoon and his tummy was a little bit harder (so he was bit grumpier than usual) but then today he's fine).
We are persevering with it though. Olly is eating moderate fructose foods but I just cannot put on him only low. There seriously would be hardly anything left for him to eat. I need to balance his need for food and nutrients with getting to the bottom of this.
What are the moderate ones? It's like Eloise said above - if the's eating just 1 food that is too high for him, then the whole thing will be null and void.
I think you need to give it a really good go despite how dificult it will be, so that then you can say you've done it and it is or isn't the problem and move onto the next thing if necessary.
J is totally fine with banana. He can eat a whole one but thefartingpear.com suggests that some people can't tolerate that much - maybe only 1/4 or 1/2. I know snacks are a big pain in the neck! I give J crackers (lots of different ones but they are all boring! rice crackers, puffed rice cakes, corn thins, corn cruskits) plus quinoa rings (I found some that contain pear juice for flavouring - so I DON'T buy those ones!!), corn puffs, pop corn.
I think I would feed him this for a few days:
Breakfast: porridge or whatever you replaced it with, berries if you want
Lunch: GF toast with berry jam, avocado
Dinner: meat and potatoes, custard made with oat milk
Snacks, crackers, popcorn, blueberries
After a few days you would know if it's fructose or not, then you can start adding other things back in (like maybe 1/4 banana) to see what his tolerance is. You can always do different meats and different styles of potato for variety. Olly probably won't even care (my DD only ate 7 foods from 6-14months - it was definitely me who minded about the lack of variety more than her!).
Anyway, just letting you know I'll happily hold your hand if you do try low fructose!