If your cousin developed this immediately after the accident, he must have had some brain injury during that accident which will take time to recover. But if he has developed it later this could be a sign of brain stroke. I recommend getting checked up by the nearest Neuro doctor.

Recovery depends on the size and site of a blood clot and also the Glasgow coma scale at the time of admission. Small blood clots in case of mild to moderate head injuries recover with medications without any surgical intervention. But strict monitoring of patients in the neuro center is advised.