My son't hair stinks (fish or sulfur-like). Shampoo doesn't help. What is this?

My 2 year old has a smelly head. It's "fish or sulfur-like". I've washed his hair with expensive, cheap and even dish soap... but it's still there. AND, it's worse when it's wet. I'm at the point of trying Tomato Juice (if it gets out skunk smells on a dog, maybe it'll help my son?). Any ideas on what to do? Does he need a doctor or a hair cut???? HELP!!!