In such cases, memory gradually improves over a long period of time, usually over a year. However the extent (that is how much he will regain) and time frame is not predictable. His Brain MRI scan should be done to know what all areas in the brain are injured / affected, and based upon that some predictions can be made.
He should also see a neuropsychologist to assess different parts of memory and other mental / cognitive functions of brain, and to know what specific mental / cognitive training he requires for better and speedier recovery. He may need to relearn the forgotten things. Some medicines like piracetam, citicoline may help this recovery, but these should be used only after a Neurologist's consult.