I sure hope his memory will eventually come back!!!
I think there is a good possibility the memory will never come back. My wife experienced a shoulder seperation and a concussion in a skiing collision more than 25 years ago and still has no recollection of how it happened.
I once had an accident resulting in unconsciousness followed by amnesia. At first I couldn't remember anything from the prior month. Over time I began to recover more and more memories but it took a few months before I could remember the accident itself. I'm not sure how memories work at the physical level, and I'd imagine the prognosis varies greatly depending on severity, but I guess in some cases the brain can find new paths to older storage.