My daughter (now 7) and son (now 6) did this for a while. I have blankets and a pillow set up next to my bed, so if they woke up during the night, they just slept next to our bed. My husband, also a night owl, was never thrilled about them coming into our rooms.
On a recent trip to visit relatives, my daughter did not sleep well. I gave her Benadryll (she can handle it but brother can't) for a week to reset her and it worked very well.
Other things that seem to be working for us. Protein snack before bed time. If they want cereal, I slip ground up sunflower seeds in the cereal before pouring the milk. Protein keeps the blood sugar from falling during the night. When the blood sugar falls, a child can wake up througout the night. Magnesium and calcium in foods or supplements help a child relax and sleep.
If his stomach is hurting, then it could mean that he's constipated or is allergic to something he ate.
If you need any more suggestions, let me know. I lost a lot of sleep when the kids were younger, so I researched all this so I could achieve sleep. However, I do tend to think holistically to a certain extent, so just keep that in mind. I prefer vitamin C over allergy meds.