I, too, have an 8yo with overflow soiling. He has been soiling on and off since toilet-training. He complains of nausea and stomach cramps, but only ever at bed-time. He was dairy intolerant, but may have grown out of that by now. He tested negative for coeliac (a family member has it). Eventually a different GP diagnosed constipation. I was gobsmacked. Ds has an excellent diet, a fabulous appetite, and is active and healthy. We had no idea that he was constipated! We even thought it might be attention-seeking behaviour.
Ds has been on Movicol on-and-off for a year so far. At first it made no real difference. But a few weeks ago we finally saw a specialist constipation nurse and she explained the whys, hows and whats of the whole process. Apparently I had been dosing ds incorrectly and managing his toilet trips incorrectly. I changed what we were doing and within a week ds had far less tummy trouble and the soiling had reduced from 0-3 times a day to 0-3 times a week.
Apparently the nurse had always been available, but because I appeared to be coping, we had not been offered an appointment!
Ds will need to be on Movicol for at least as long as he has been constipated. In his case this could mean 4y or more. He needs to have regular disimpaction when the Movicol dosage is greatly increased for a week. Holiday homework!
I also had a constipated toddler (different ds) whom we managed with diet. Increasing wheat fibre was an utter disaster: caused more pain and more tummy discomfort. Increasing oat fibre OTOH was very helpful. So we switched from whole meal bread to white (naice white!
) plus oatcakes and porridge. Dried fruit of any sort was also very helpful. Fortunately ds1 loved it. I also cooked with dried fruit; not just in sweet dishes, but in meat stews as well. Meat was actually a surprise: reducing the amount of red meat in his diet also helped. It wasn't just that he had more veg to compensate. There was a distinct difference in his bowel movements if he had, say, fish stew rather than lamb stew.
Increasing fluids was tricky, and probably also the source of his problem. Homemade juice ice-lollies were a hit, as was jelly. I even learned how to make jelly from scratch, so that he could have pure juice jelly with no added rubbish. Yeah, I know, PFB.
The constipation resolved at about 2 1/4, and he toilet-trained very easily a few months later. He is a log-laying, loo-blocking, healthy teenager now.
Sorry for long screed. Hopefully something in my boys' experiences will help you. Please get a referral to the Cobstipation Clinic, and see them in person. They were vastly more helpful and informative than any of the GPs or paediatricians we saw. A matter of expertise, I expect.