OP you can get a genetic test to find out if you're DS is more predisposed to Type 1 diabetes. https://www.elsadiabetes.nhs.uk/ It's a trial that's ongoing in the UK run by the NHS.
I would have done it for one of mine but DC was too old at 14. I understand your worry.
Here also is an interesting medical research report: https://pubmed.ncbi.nlm.nih.gov/35045871/
HAMSAB (high-amylose maize-resistant starch modified with acetate and butyrate) is not commercially available, (the stuff used in this trial) nor are we mice so take the study with a pinch of salt but it's something to consider.
Anything that increases short chain fatty acids are a good thing for any human!
Foods That Promote SCFA Production:
Fruits: Apples, pears, berries, oranges, and bananas are good sources of fiber.
Vegetables: Broccoli, carrots, spinach, kale, and sweet potatoes are excellent choices.
Legumes: Beans and lentils are rich in fiber.
Nuts and Seeds: Almonds, chia seeds, and flaxseeds provide fiber and healthy fats.
Whole Grains: Opt for whole grains over refined grains for more fiber.
Psyllium Husk: A fiber supplement that can be added to the diet.
Popcorn: A good source of fiber, especially when prepared with minimal added butter or oil.
Fermented Foods: Fermented foods like yogurt, sauerkraut, and kimchi can also contribute to SCFA production.
Resistant Starch: Foods like potatoes (especially when cooled after cooking), unripe bananas, and cooked and cooled pasta contain resistant starch, which is fermented by gut bacteria to produce SCFAs.
Good luck OP x