Yeah, he recieved a hemispherectomy. I believe it's only effective on younger individuals, as the brain is still in a developmental stage and the remaining hemisphere can pick up the slack [with time]. It's not exactly a common procedure, but it's used from time to time to treat epilepsy, such as if a child is experiencing especially violent seizures.
I've always wondered if they just leave the rest of the skull hollow without any reinforcement. I mean, does the presence of the brain support the skull at all, and can it survive perfectly without anything under it?