I think it’s probably some kind of IBD as he’s not got any other symptoms of an autoimmune condition, so I’d push for a colonoscopy as the prolapse also needs looking at.
If the blood test comes back positive, I’d ask them to do some further investigations just to rule out an autoimmune condition.
Re anxiety and IBS it’s not just people with autism that suffer with anxiety that can subsequently trigger IBS, I have IBS and anxiety and certain foods trigger it in me (anxiety is my main trigger). I know you’ve done the food diary and cut out certain foods, but I’d also look at foods that can trigger IBS.
I’m not sure what his diet is like but it’s thongs like:
cut down on rich or fatty foods including chips, fast foods, pies, batter, cheese, pizza, creamy sauces, snacks such as crisps, chocolate, cake and biscuits, spreads and cooking oils, and fatty meats such as burgers and sausages
reduce your intake of manufactured foods and cook from fresh ingredients where possible
limit fresh fruit to three portions per day (one portion is 80g).
If symptoms include bloating and wind:
• Limit intake of gas producing foods e.g. beans and pulses, Brussels sprouts, cauliflower, and also sugar-free mints/chewing gum.
• You may find it helpful to eat oats (such as oat-based breakfast cereal or porridge) and linseeds (up to one tablespoon per day).
If symptoms include diarrhoea:
• Replace lost fluids by drinking plenty.
• Limit caffeine intake from tea, coffee and soft drinks to three drinks per day.
• Try reducing intake of high-fibre food (such as whole-wheat breakfast cereals and breads).
• Avoid sugar-free sweets, mints, gum and drinks containing sorbitol, mannitol and xylitol.
So things like sugar free cordial can be triggers, I know if I have sugar free sweets that sets me off! 😂
www.bda.uk.com/foodfacts/IBSfoodfacts.pdf