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It might not be a bad idea to have someone take a look at him as his injury could be serious. With a sudden injury to the neck there are several things that come to mind but with his voice affected I'm concerned about an injury to the pharynx (throat) or even the laryngeal nerve that could have caused paralysis of his vocal cords.
I would have some one look at him soon though if he is not having any trouble breathing or swallowing it is less likely he has a spinal injury but he need to have his reflexes and cranial nerves tested an any physician can do that.
If he is not having any pain or numbness in his arms or legs, difficulty walking or using his fingers and hands, breathing and swallowing ok, then you can just observe for now as he might just have what they call in sports a "stinger". If he begins having any of the above symptoms then take him in immediately.