Hi E.,
It may very well be a flu or intestinal bug like gastroenteritis which is very uncomfortable,but my daughter had a similiar problem where she was eating things that would make her cramp up terribly from gas. Is he passing alot of gas?
Has there been a change in his diet,or since it's around the holidays,maybe he ate something that is not agreeing with him,and it's causing him pain.Maybe he's developed a lactose intolerance.
If your instincts are telling you diffrent,there are certain things they can't diagnose with an x-ray or blood work.Go get another opinion if you're not satisfied with what you're being told. There are things like intestinal blockages,Chrons' disease, diverticulosis,all these can hurt.Demand them to do a sonogram or to see if the diffrent parts of your sons colon are normal. My daughters wasnt. She was severely constipated,but we didn't know that because she was making small bowel movements. Three parts of her colon were abnormal. She's now 11 and she's fine. I am not saying this could be the problem,they're just possibilities the doctors dont mention. Follow your instincts,and if it continues,definitely pursue by any means necessary.Also if he starts vomiting and develops diarreah and runs a fever,have him checked for appendicitis.
I wish you luck and i am sure everything will work itself out.
~R.~