After head injury
, if there is no bleeding from ears/ nose/ around eyes, if there is no forceful vomiting
, if there are no seizures/ altered consciousness, you need not worry.
The small lump must be a bruise
. You may give cold compresses on the lump.
Generally due to increased vascularity, head of a kid will be more warm than the rest of the body. Its normal. Do not worry.
You may give Acetaminophen
in right dosage for reduction of pain.
Hope my answer helps you.
Wish your son good health.