Boy do I have experience in this! More then I care to know. It is probably an allergic reaction. It can be food or inhalent (air borne, dust, etc.) First, you need to know that just because a skin (scratch) test or blood test comes back negative, it does not mean your son is not allergic to the allergen being tested. Skin scratch tests are not very accurate and blood tests are worse, unless you have background info to give them something to narrow down what they are looking for on the blood test.
The BEST ADVICE I got was from our pediatrician who had some background in nutrition and allergies. KEEP A FOOD AND ACTIVITY JOURNAL! Not just when you son breaks out either but every day, all the time. Write down everything he eats, in detail and I chose to write down her activities like when we went to a public playground, etc.
Here is an example of what I mean detailed:
Don't put down Breakfast: oatmeal
Lunch: Hamburger, fries
Dinner: Mac n cheese
Instead write down:
Breakfast: Cinnamon Roll Oatmeal
Lunch: Cheeseburger, mayo, white bread, ketchup (home made)
Dinner: Mac n cheese (if needed put brand, we only eat 1 brand)
Do this especially in the beginning, until you get son's eating habits down, then you can shorten. Kids are pretty limited on their eating.
Write down every break out in detail, how much body covered with hives, time, if needed Benadryl.
Then you can go back and trace what makes him break out......take to your doctor and it will help them with your son's hive maintenance.
Also, you can figure out if it's something like corn, wheat or a preservative which is in everything.
Our daughter started lightly bleeding from her bowel area 1 day before her 4th birthday. We went through heck with it, 4 doctors, possible colonoscopy, UCLA visits, then allery route, 2 scratch tests and a total of 6 full blood tests (6 viles each test) between the ages 4 and 5 1/2 and approx 1 1/2 - 2 years of on and off jounaling and several different medicines to find out she is allergice to corn, corn syrup and all derivitives (including Xantham Gum) of corn. None of the tests gave us anything. The jounal is what gave us the information we needed to help our daughter.
You can even do trials by removing some foods from his diet along with jounaling, but remember, it might be simple like bananas or it might be difficult, like corn. Corn is the hardest thing to remove from the diet because it is in EVERYTHING as some form of ingredient.
sorry for the long response by I am passionate about not having other children and families go through what we did.
I went through a terrible time with my food allergies too. Both my daughter and I food allergies affects internally. I spent years being diagnosed as having GI problems. I had 3 upper GI tests, blood tests, and was on several different GI meds. almost 10 years later, found out I was just allergic to wheat and a few other things. No more stomach problems.
Just keep journaling! It will help you tons!