I don't think you are being overly concerned. It is very likely that the head injury caused the blurred vision. Vision problems are one of many symptoms associated with head injury. If he still has symptoms associated with the injury, that means that the brain has not healed. I would be cautious and not allow him to play sports until complete resolution of symptoms. If his brain is not fully healed, another hit will make things much worse. You should at least restrict his activities until you see the neuro.