This also happened to my DS2. Just had a dentist look at it last week. All they said was to monitor it as they can't do anything unless you want them to remove it which is obviously traumatic for a child. Need to take him back if it starts to hurt or an abscess forms.

DS2 had a fall about 2 months ago. His front tooth is slightly grey now but does not hurt. Dentist said it may have damaged his adult tooth but we won't know til it comes through. We can "fix" it then (ie.braces, plate etc). It is only a faint grey but I can see it. DS2 doesn't seem to mind yet. Hopefully the greyness will fade or it will fall out early. He is 4 but no teeth are loose and I do hope he keeps his baby teeth as long as possible. I have heard the longer it takes for baby teeth to fall out, the stronger the teeth. Hope that is true.