L.L.
For people with Celiac disease, they absolutely feel better almost immediately...because their body is having an autoimmune response to the gluten in foods. A sensitivity may take longer to show improvement, but the big issue is whether or not he is truly gluten free.
You have to be fairly vigilant at first in determining what is truly gluten free...it's not just about eliminating obvious sources of wheat from your diet. Many processed foods use malt, which is generally made from barley, and anything that lists "flavorings" or "spices" or "seasonings" may contain wheat, barley, rye, etc, as an ingredient. Although foods made in the US are held to a higher standard of labelling, imported foods are not. For example, Klass makes a rice drink mix that says "Gluten Free!" on the front and clearly says "Contains wheat" on the back.
You also need to look at your cookware and other sources of contamination...the biggest one is the toaster. Putting GF bread in a toaster that was EVER used as a non-GF toaster is contaminating your food, as can be making cookies on a non-GF baking sheet, etc.
Lastly, the source of his diarrhea may not be gluten and if that's the case, then of course he isn't feeling better...but I hope he does soon! If you'd like any help with a gluten free diet, please feel free to PM me. I do have a child with CD so have learned to be absolutely rigid.