Soph, first step for a gluten intolerance is to have a blood test (testing for gliadins in the blood stream I think) if this comes back positive your son will need to have a gastroscopy to confirm the diagnosis although a peadiatrician might suggest a gluten challenge as a non-invasive form of diagnosis it won't diagnose whether he has coeliacs disease or not.

Symptoms for coeliacs disease are contradictory (eg diarrhoea and constipation are both symtpoms) and not alway obvious.  Check out this site for further details and see if any or all of it matches (nb. coeliacs don't display all symptoms) http://www.colourcards.com/coeliac/

I wasn't diagnosed until I was 26, I was not underweight and many of the symptoms I had were mild and other I would have never put down to gluten in my diet for eg I was on condroitens for joint pain (used by arthritics) by the time I was 24.  In hindsight I suffered from it for a long time.

The tests to diagnose are invasive but I had gene test done on Michaela a couple of years ago and she carries the gene so I will have her tested when she's older (nb. it's very hard to diagnos in young children because generally they haven't consumed enough gluten to cause major damage to the stomach yet) docs prefer to wait until kids are 3-4yo unless their symptoms are obvious.