My four year old has had one
shampoo in his life: when the nurse at the hospital bathed him the day he was born. The only thing we put on it is water, and his hair is just beautiful: we don't really even get it wet that often--maybe a couple of times a month. We also use minimal soap on his skin (he generally bathes twice a week), except for handwashing.
His hair is even nicer than mine, which still gets occasionally greasy--though I'm a chef and exposed to plenty of grease at work. I've been no poo (water only) for about six years now, and like my son, I rarely even get it wet; I use a vinegar rinse to help with my grease about once a month. Once or twice a year, if the build up is really bad, I use a little bit of soap in my hair. But I've never had to do that in my son's hair.