Dear ! at 12 years of age one have mixed dentition (means both type of teeth are present in mouth milk teeth as well as permanent teeth) so, future treatment will be decided on" which tooth has been extracted milk tooth or permanent one.
1) milk tooth is extracted, so permanent tooth will erupt soon as replacement, that can be confirmed by dentist
after examining an x-ray to see the location of upcoming tooth.
2) if permanent tooth has been removed then no replacement tooth will erupt in empty space,
so, you must show your son to a dentist, who can clarify your doubts after seeing the case.