Maybe due to some trauma in front teeth.? Can go for xray No need for any treatment until there is pain or any inflammation around the tooth.

I agree with Dr Uma, It looks like injury related discolouration. Blood circulation to the particular teeth gets hampered. the blood clot and tissues trapped inside the tooth causes this discolouration. Nothing to worry as this tooth eventually gets replaced. It is an internal discolouration do not try to brush too hard on it. No matter how hard you brush It will not help.

Has there been an injury to this particular tooth? It seems like there has been some sort of an injury by either being hit with something or maybe he fell down and hurt this tooth. As of now there's nothing to worry about. Would suggest you to go to a pediatric dentist and get this tooth assessed with an x-ray. No treatment is needed until there's pain or a gum boil above this tooth. It can be simply monitored until it's time of shedding if all goes well!