My 18 Year Old Son Is Complaining Of Minor Abdomen

My 18 year old son is complaining of minor abdomen pain, in the top left. An ache or pinch and sharp at times and minor back pain around his Kidney. The pain comes and goes. He has no fever, vomiting or blood in his stool. He has had Diarrhea once or twice a week for the last few weeks. No pain or blood when urinating. Has no loss of appetite, but thinks he is more thirsty than normal.