My five year old son with T1D has a tummy ache so often. Does anyone else with T1 have this problem? I would say it happens at least once a week. He won't want to play, sometimes he cries and just holds his belly. I've checked his blood sugar and his keytones, and nothing seemed out of the ordinary.
Time for a doctor visit. There are so many things it could be. Maybe you could keep track of what he eats in addition to the carbs and see a pattern.
Definitely call the doctor. People with T1D do end up with gastrointestinal problems. Not saying your son has those but, they can do tests. Have him tested for celiac as well, which is when people are allergic to gluten. That sometimes can be the culprit too. It's hidden in so many foods. It has been tied into Type 1. Hope your son feels better soon!!!
Could it be related to stress? Sometimes I find that stressors in my life affect my stomach. And people with diabetes can have extra stresses. Perhaps ask your son if there is something he is worried about? If this is the case you can teach him some relaxation techniques like deep breathing which can help.