HI 
Thanks for posting your query. 
   There are many causes of Stomach Pain. Common are :
1. Hyperacidity/Gastritis.
2. Ulcers in stomach or Intestine. 
3. 
Pancreatitis ( Inflammation of Pancreas )
4. 
Cholecystitis ( Inflammation of 
Gall Bladder )
5. Kidney stone 
6. Inflammation of Appendix. 
7. 
Pelvic Inflammatory disease.
     I need more information like  
1. Is the pain continuous or intermittent. 
2. Does intake of food increases pain?
3. Is his motions proper?
4. Any Urinary problems ? 
    Get an 
Ultrasound Abdomen pelvis , Urine routine test and revert back with the scan report and the details asked . 
Hope this information is useful to you. 
Any clarification feel free to ask.