What are some possible reasons seemingly healthy 22 year old male could fall into a 2+ week coma?

Hard to tell. Coma must have a cause/reason for it. The fact the person you are describing was seemingly healthy doesn't mean he fell into coma without a reason. We all know that strokes, for instance, may occur in a matter of minutes resulting in coma in a seemingly normal person. A ruptured brain aneurysm is a common cause of intracranial hemorrhage that may result in coma.