M.L.
Paul Mitchell and meleluca both have a great Tea Tree shampoos, though they may be even more expensive.
I'm a hairsytlist and I agree, I love using tea tree. I would maybe get something less expensive for your son, like the Sally's brand. There are actually a lot of tea tree shampoos around the $5 range. Go to an Ulta or Sephora and you will find a lot. Sally's usually have Paul Mitchell generic brands.
http://www.google.com/#hl=en&sugexp=gsish&xhr=t&a...