I would definately second the last two posts from Jilly and Jacqueline--check for environmental causes. I had giardia, got good and sick, never got it cured, and it nearly killed my son after he was born because he got it from me. Also, DO NOT use any type of fluoride--toothpaste, water, etc...as it is a neurotoxin, among other things, and no matter what they tell ya--it ain't good for teeth.
The other thing I would say is that if your son is just having a challenge getting rid of a virus, some herbal supplementation may give his immune system the boost to get rid of it. Vomiting and diarrhea are the body's way of getting rid of something bad. Organic grapeseed extract is something I've given my kids in the past. I just break open the capsule and sprinkle some on some icecream or yogurt (or applesauce, etc...) It won't cause side effects, and it really helps, being an antioxidant.
Ok...please don't all think I'm crazy. When my son was sick, I couldn't get the Dr. to listen to me. He kept telling me it was because I was nursing and a first time mother--the baby must not be getting enough to eat, so that is why he was so unhappy and losing weight. I took a fecal sample from his diaper to a veterinarian, and told them I thought my "puppy" had a parasite. The tech told me, "Oh, honey, he has giardia--I can tell just by looking at the baggie. I'll go run the test, but I've seen it a thousand times..." Sure enough, it was. I went back to a different Dr., pissed off and with test results in hand, ready for battle. He calmed me down ...lol...didn't treat me like a crazy woman, gave me a prescription right away for a anti-parasitic drug--problem solved, baby immediately became happy and gained back over a pound a week--and is now a healthy, handsome (well, I'm his momma, so I get to say that...lol...) 9 year old.
So--there's my story. Trust your gut, check your environment, do your own tests if need be.
Best wishes and blessings to you and your little guy--I'm absolutely delighted to help if you want to contact me.