There is no need to worry; your son will probably lose his front baby tooth at around six years of age, just before the permanent front tooth comes into the mouth. The grey colour is the result of some bleeding that has occurred into the baby tooth after receiving the trauma. The grey colour may be a temporary or a permanent feature, depending on the status of the tooth. Sometimes, the nerve of the tooth dies following trauma. This would be a good time to take your son to see a dentist who could advise you whether the tooth is dying or not and if any intervention is required.