I am not a medical professional and am basing my answer purely on experience. I have suffered from severe stomach ache, resulting in admission in the ICU. However, this happened to me as an adult. All tests were negative, but the pain was periodic and unbearable.
Finally, a doctor recommended that I do a food allergen test. Before the episodes, I was aware that I was allergic to sea-food and avoided it. Anyway, a patch test revealed that I was allergic to gluten, which is naturally present in wheat. Your child's problem could be food-related.
In India, we do not take food allergies seriously. Gluten-intolerance is quite common and can cause stomach pain and gastric problems. Other common foods that people can be allergic to include milk and all dairy (lactose intolerance), peanuts and other nuts, eggs, chocolate, brinjal, fish, soy, and fish and prawns etc.
I advise you to either get your child tested for allergens or keep a note of what he eats to identify the triggers and avoid them. There are different types of tests available, and it is best to go for a test when the child has the pain.
If you can remember what the child ate, within the last 24 hours of the episode, it could help establish a pattern. Maintain a food diary and jot down everything the child eats. Ask the child to not eat anything outside, even from a friend's tiffin.
The other reason for stomach-ache could be psychological. Is the child under some stress? I don't want to scare you, but, I am saying this from experience, having worked with children, child-abuse can also manifest itself into stomach aches. It could be bullying by peers, or physical, verbal and sexual abuse. Is the child's behaviour normal, or is the child withdrawn?
I am hoping that you find a solution to the problem and that your child recovers soon. Good luck.