I wanted to chime in with a thought that many people do not know about. I was diagnosed with gluten intolerance in fall 2008 by an alternative doctor. I wasn't really told much about it or what I should be doing, so much of what I know has been from my own research efforts.
The typical symptoms, resulting from malabsorption or improper digestion of dietary nutrients, include abdominal bloating or pain, diarrhea, constipation, gaseousness, or nausea with or without vomiting. It appears that acid reflux in the esophagus, manifesting as heartburn, may be a potential symptom as well. Other symptoms people experience include fatigue, joint pains, mouth ulcers (this includes both canker sores and cold sores), bone pain, abnormal menses in women, and infertility. And these are just some of the symptoms; there are many more.
The fact that your son started displaying cold sores when he was 1 years old--right at the time when he started eating solid food and probably foods containing wheat, rye, barley, and oats--warrants, I think, a look at whether or not gluten intolerance (or Celiac disease) could be the cause, especially if he's as susceptible to cold sores as you say he is.
People's bodies display their intolerance against wheat gluten/gliadin proteins in different ways. Some have diarrhea; others have constipation. One lady got hives all over her body anytime she ingested anything with wheat proteins in it. Another broke out in (acne) Rosacea whenever wheat products were consumed. And then there are some people who don't show any symptoms at all.
Since gluten intolerance/Celiac disease attacks the small intestine, if a person has it and it isn't caught, the small intestine becomes inflammed/destroyed, which can result in malnutrition and malabsorption of essential vitamins, minerals, electrolytes, amino acids, etc., etc. That happened to me, and at the moment, I'm taking lots and lots of supplements to build my body back up. Besides the nutritional problems, gluten intolerance/Celiac disease can also cause liver and kidney problems, as well as system wide autoimmune diseases.
Who knew? Definitely not me.
You can try a tradional doctor or gastroenterologist, but since they're trained to wait until there are specific symptoms before they'll do anything, 9 out of 10 times, they'll say there's nothing wrong with you--even though you're gluten intolerant. Or they won't even test, which is what happened to me. I saw an Integration doctor, who tested me. I was surprised at how high my F14-Gliadin Ab, SIgA test result was (81 units; anything over 15 units is positive; the average score is around 45 units).
The simple way to cure it? Simply through an absolutely gluten-free diet. No prescription drugs needed! And, if you follow the diet faithfully, the cold sores and other symptoms should disappear as well.