Our allergist said that blood testing isn't very accurate until 3 or 4 yrs old. And If you're dealing with IgG reactions (which are just as serious), they won't show up on a RAST test (only IgE).
Have you not done just a regular ED and/or food journal? I would do one and for a month removing gluten as well as the top 8 plus corn (or maybe just wheat, soy, dairy, corn, and possibly eggs). I don't think that you'll see significant weight gain right away, but you will see an improvement in the diarrhea if it's any of those foods. After a month, you can always add back in the foods one at a time (one a week or so) and see what happens. It just seems like you'll waste time if you JUST eliminate gluten and it turns out to be more than gluten or something different. Then, you'll have to do another ED and another trial. Two birds and all that.
Also, if you're nursing, you need to strictly the foods as well to get an accurate reading.
One more thing, dairy is the number one food sensitivity. I'd be VERY surprised if your child is having these issues and dairy is NOT one of the culprits. Even celiac or gluten intolerance usually induces dairy protein intolerance (usually temporary) because of gut damage.