TODDLER TOOTH TURNING GREY

so my son fell a couple weeks ago and really did a number on his lip. he bit down and tore through his lip. luckily it healed perfectly. now the problem we’re having is his one front tooth is turning grey. i read it could be a sign of trauma to the tooth or lack of oxygen... has anyone else had this problem? what did the dentist do? did it heal on its own? HELP!!!