Have you purchased any new furniture, flooring, cleaners etc?
If you have not you may want to try a pediatric dermatologist which they can do a skin (flake) testing to see what it is and from there can determine if this is due to an allergy or some kind or something else. But that may be the only way of knowing at this point. Which I do agree, treating is fine, but you need to find the cause to cure this not treat it. So I would start with the dermatologist.
Hope this helps, If you need anything else please feel free to ask. If you found this helpful don’t forget to click the *accept* button!