Could A Toddler S Hair Smelling Burnt For 2 Years, Even After Washing Be A Medical Or Genetic Issue?

Hello, so my son is 4 and for the last 2 yrs his hair has a burnt smell, even after her showers, obviously the smell of the shampoo can almost overpower the smell at first and I’ve tried many babyshapoos regular shampoos and I guess I’m just curious if doctors see this at all or rarely, if it’s a medical issue or genetic, or is it a symptom of something. I’ve never used heat tools on his head so it’s not actually from being burnt.