hello and welcome,
your son is suffering from 'localised periodontitis'of gums.check
whether your son has a history of fracture/gum infection/decayed tooth.
it can also be due to poor oral hygiene
accmulation food debris called plaque
has formed around the marginal gingiva.due to absess and pocket formation,there gingival recession
occured ,which has caused the receding or lowering of gum level.
thorough scaling and root planning has to be done to remove the plaque around the tooth.
if the tooth is mobile heavily,then splinting of tooth should be done.
if tooth is slightly mobile,it will become firm once the gum infection subside and the gum becomes firm and resilient.
use soft interdental brush
and dental floss to clean the teeth.