Hopefully he is doin well now. Head injury or any other bodily injury causing severe pain can trigger a condition called SIADH , which result in excess water retention in your body, and reduces Sodium content in our body. Sodium is an essential
electrolytes for brain activity, when it reduces it could result in all the symptoms you have mentioned. Treatment consists is water restriction , salty diet etc , however severe hyponatremia is an medical emergency and has to be treated at a hospital by a competent doc !