"My question is considering the primitive medicine of the day, how would this wound have been treated? Would the doctors have placed some type of plate in his head to cover the wound?"

While I can't comment directly on your ggrandfather's case, Medical care during the war is not as primative as we would be lead to believe from many of the common impressions, such as the "meatball battlefield surgery" of sawing off legs and arms by the dozens. True they didn't have the type of medical practises that we do today but for their day a detailed study would surprise most people at what they were able to do.

Yes they did do brain surgery and they did put plates in the head. Silver was a material that they used for this purpose. Coins where often used. The normal surgeon kits would carry a Trepaining kit/saw for boring holes in the head.