An isolated positive IgG gliadin is often the first sign of a gluten sensitivity, which is more common than frank celiac.

It does indicate that the body is making antibodies to gluten.

The transglutaminase result is the one most often associated with the villous damage from frank celiac.

It would seem prudent to see if you can get him on a gluten-free diet. Gluten antibodies can have effects well beyond the digestive tract--in fact, Dr. Hadjivassiliou has opined that people with positive IgG gliadin results are more likely to have initial neurological manifestations.

Have you taken a look at Jcc's Gluten File? It's located at the top of this board in the Useful Websites area, and is the best database on the whole subject of gluten out there, and explains this all very well, with references to the actual medical studies and links to the papers.