Hi! It is very difficult to say what is wrong. Dizziness could be due to several causes both related and unrelated to injury. There is a chance that your cousin is depressed after the injury. If his cognitive skills are fine then he can attend school. Again as I said, it is difficult to say what the problem is.
I had a mild Traumatic Brain Injury and severe concussion 1 year 8 months ago. Post Concussion Syndrome followed after that and that is what probably your cousin's doctor call it.
I have several symptoms and the worst is dizziness, heavy headed and fatigue. Been to many therapies, meds and no help. Right now I am trying some PT for my neck which seems to be causing my dizziness. I don't know and no one knows including specialists. It is tough to diagnose dizziness. It can be malfunctioning of ear, neck, brain, blood pressure and lots of other things.
Probably your cousin must have been to many specialists. Ask him to try vestibular therapy, cranio sacral therapy, neurofeedback and there are plenty of other therapies. I am also doing some controlled exercise on bike and key thing is not to push too much. If you are in Toronto you can consult Dr.Leddy at University of Buffalo concussion clinic. PCS is hell and hope we all can get better