Thanks for posting your query.
The scalp have an abundant blood supply and injury
to these areas often results in bleeding under the skin. When the bleeding is restricted to one area, it causes bruising and swelling.
It sounds like just a soft tissue bruise
and not a more serious, sub-dermal hematoma
. The good signs are him acting normal and not showing any sensitivity with it...however you can check for a concussion
by shining a flashlight in his eyes and watching the dilation response of his pupils...the response should be quick and the pupil should get small in the light and open when you pull the light away. If there is not normal pupil repsonse that is a sure sign of a concussion. Good thing to take him to the doctor to check it out...
Also seek emergency attention if your child exhibits the following signs or symptoms following a head injury:
Persistent or worsening headache
Hope I have answered your query.
Wishing you good health.