By your description it appears that your husband underwent a strip procedure which has left a 'smiley' mark on the back of his head. First of all you should be very clear that no technique is currently available in the world that can make the mark disappear. Having said that, I must also tell you that this scar can be made much less noticeable even when your husband keeps short hair.
The most useful technique in this regard is FUE or body hair implantation
in the scar itself. This way non hair bearing scar can be converted to hair bearing scar which is much less noticeable. The second technique would be scar revision and trichophytic closure which should be planned if the scar is very wide. Third would be scalp
micro pigmentation in which small hair shaped tattoos are made on the scar to mix it with tonsured scalp.
You can take help of your hair transplant
surgeon in this regard. Feel free to ask further about these options.