K., I am having the same problem with my son - but he is only 2 months old and barely has any hair yet, just "peach fuzz". My other two children (girl and boy) have never had this problem, so I was completely taken aback by it with this child of mine! His scalp has this terrible "dirty hair" smell, but obviously it's not from his poor hygiene or even his hair ... since he has none! His scalp doesn't feel or look oily either, so it has been very baffling to me. In his two short months of life so far, I have found that using a strawberry-scented conditioner helps keep the bad smell away for a little longer than if I only use baby shampoo. (I'm sure there are many other hair products that could similarly help ward off the smell for awhile.) Even so, the smell ultimately returns within a few hours after I bathe him.
My husband's scalp has a tendency to develop the same smell, as does my husband's brother, seemingly sooner than most people. For example, my other two kids and I could go for days without washing our hair (not that we'd want to!), and I swear it wouldn't smell, but by the end of a day, my husband's hair will smell bad, even though he showers every day. That has caused me to think that there must be some genetic aspect to the problem. I've been trying not to obsess over it, but the top of my little guy's head definitely smells bad almost all the time, so it's funny that you are posting this question that for me is very timely!!