Your boyfriend has post traumatic alopecia
. The cause could be either the scalp injury
itself or improper wound closure.In any case this loss is permanent. Medical therapy is of no use.
The only way to grow hair in that area is through hair transplantation
. Second option would be to remove the scarred area and stitch the hair bearing edges together. He can also use existing hair to cover the bald spot if the area is small.
I hope you found this answer useful. Feel free to ask more if you need.