His neurosurgeon should be able to help you with this. A neuro-rehab clinic will also have good information for you.

From my basic understanding, the craniectomy was done to allow the brain to swell without endangering the brain. The flap will expand under pressure. Once his brain has recovered from the trauma, they can close up the skull opening. It can take quite some time for a traumatized brain to settle down.

There is a system in the brain that regulates pressure. This system can get injured and can be slow to heal. This system can be injured in a simple concussion. A second concussion before the system recovers can result in Second Impact Syndrome that can be fatal if not treated immediately.

As I said before, his doctor should be able to elaborate further.

My best to you and your father. You are blessed to still have him.