The bump over the head after a fall is due to accumulation of fluid or blood (hematoma
The blood cells accumulated are eventually broken down and absorbed in the body.
But this is a slow process & takes times to disappear completely.
Till that time, symptomatic relief is obtained by giving analgesics, ice compression & adequate rest.
If it still persists or if he shows any other associated symptoms like vomiting
, loss of consciousness
, convulsions, etc...then it will be better to consult the Pediatrician
Wishing your son a good health.
Thanks & take care.