You should probably see a doctor to have his glucose tested and then bring up your concerns. Your doctor can refer you to an allergist who can do the testing on your son and find out if he is allergic to these different things. Hope everything works out. I will keep you both in my thoughts and prayers.
Unfortunately, it sounds like it could be a gluten/wheat allergy since it's digestive & skin. The blood tests aren't 100% accurate, so you might want to try cutting gluten-containing foods out of his diet and see if the symptoms go away. Even though avoiding gluten is tough, it is worth it to feel better. I was sick for 46 years because nobody figured out what my problem was. If you are allergic to gluten and keep eating it, you can get immune system diseases. There are many different possible immune diseases, like thyroid problems, lupus, scleroderma, many others.
Hello Laddybugg~I would suggest seeing an Internest,they go deeper into problems.
As to the rash and diarrhea, he could have a sensitivity to fructose or any sugar substitute. There are food allergy tests available, and I would say that should be the next step, the ones I have had all involved just drawing some blood, then the lab tests it with different foods, this will certainly tell what he is and isn't allergic too. It is called the RAST test, I would ask the doc about it the next time you see him/her.
Oh, I almost forgot, regarding it being a GI issue, well, it could be, I usually go to the health food store and puchase digestive enzymes, these will help with his digestion, and I have read that some allergies are indeed caused by improper digestion. Also, take some Probiotics, this will put the friendly bacteria back in his gut that the diarrhea is taking out.