I was a junior doctor and not practicing any more so definitely not pro on this, and I'm reluctant to give advice without actually knowing the full history - but I know the state of the NHS at the moment and how hard it is to get some thorough centralised advice, but please take this all with a pinch of salt and check this all with their GP! We did learn a bit about this at med school: if you'd said diarrhoea and tummy pain I'd think much more about intolerances and suggest doing a food diary to work out the food trigger(s), but since you specify constipation (not diarrhoea) and ND, the cause sounds to me much more likely (but not definitely) to be psychological - so I'd also consider speaking to an occupational therapist who works with ND kids, or a psychologist specialising in ND, maybe even with a special interest in "organic" or gastrointestinal conditions in ND, if you can find one.
Have you considered things like making sure there's a toilet your kid feels comfortable using every day, and making sure they have lots of time when they do go to the toilet, and putting things in place to help them to feel relaxed on the toilet and beforehand? (Lots of ND kids won't go in unknown toilets/school toilets, etc, which then causes constipation - actually most young children and even many adults have trouble doing a poo in unknown bathrooms but it's hardest of all for ND kids). And carving out plenty of time every day specifically for them to go in a toilet where they feel comfortable? Also making pooing a routine (same time and place every day), at a time of day when they are relaxed and don't have any time pressure, which can train the body to go at the same time each day (best is after a big meal when there is no time pressure). I think there are other things you can do to make it easier for them to go to the toilet, but am not an expert on it - worth a google. It's a good idea to talk with your kid about how they feel going to the toilet in different places - eg at school, ask if they find the toilet ok, if it makes them nervous and why, if they find the toilets there gross or dirty, if the toilet paper is too scratchy and uncomfortable, if they worry they don't know how best to clean themselves afterwards, or if they are nervous someone will hear them, etc, if they prefer going at home and why, that kind of thing - to then help you come up with some practical solutions in case it is because they don't feel comfortable/it's due to a sensory sensitivity or anxiety.
Also, are they eating and drinking enough? If they aren't eating enough (especially fibre) or aren't hydrated enough they'll naturally be constipated - constipation can then cause tummy ache and nausea. So could be worth thinking about changing breakfast to wheetabix or bran flakes (fibre) with lots of milk and a big glass of water (fluids), for example. Check with your GP first! On the other hand, if that makes it worse then you'd think about gluten intolerance (in wheetabix) or lactose intolerance (in milk) being the problem.
That being said, it's always important to first rule out all possible physical causes before concluding it's psychological, and neurodiversity is also associated with both "physical" and psychological gut issues too, so you're doing the right thing seeing a gastrointestinal doctor! And it would be helpful keeping a food diary, tracking exactly what they eat and drink and when, as well as tracking the symptoms and when, as well as their anxiety/stress levels and when, and sleep quality the night before - this would help you to see if there are any intolerances to particular foods, or pinpoint if stress might be the cause - this would also be really useful to bring with you to the gastrointestinal doctor appointment. Hope that's helpful, good luck :)