Are you referring to the IgA test? It tests for immunoglobulin A rather than testing 'for gluten' as such, it is looking for an abnormal and overactive autoimmune response which might indicate coeliac disease, of which raised levels are a marker. If a person with coeliac disease is actively consuming gluten it can usually be indicated in a high immune response to it as an antagonist.
As you say if they're low it's very unlikely he has coeliac disease however it can be an indicator of other conditions indicative of a lower functioning of his immune system, which most likely given he has no serious symptoms could be something milder such as asthma or even allergies.
IgA ultimately exists for immune function in the mucous membranes of the body so often affects these areas when at low or raised levels. Hence why we see changes to it result in coeliac disease (the digestive is formed of mucous membrane) or things such as the lungs where a low reading will indicate something such as asthma or present as allergies, where we see irritation in the mucous membranes of the throat nose or eyes etc.
I would associate his symptoms with eczema or psoriasis. I don't know the age of your ds which would ultimately affect the interpretation of his blood works, as would other symptoms, his full blood count and medical history.
Not what you've asked but I would source a Urea based cream in a strong % and try that on the skin if you haven't. Eucerin 10% is good but there are loads on Amazon. Apply regularly and after contact with water and keep his arms covered if out in the cold and steer away from fragrances shower gels, washing gels etc. Or try OTC antihistamines and see if this affects the itch cycle and be trying to boost the immune systems support with a good vitamin regime and diet. Do check with your GP ultimately though, especially if he is a child rather than an adult 'DS' 
Hope you get somewhere OP!