☆.A.
What does his dentist say?
My son bumped his front tooth REALLY hard and a few years later it had to be pulled. So keep an eye on those teeth and make sure to let the dentist know he's had trauma to them.
If the roots were "short" they were probably being pushed out by the new tooth anyway. I can tell you from experience that baby tooth roots are LONG when they haven't been bumped out by the new tooth coming in.