Thank you for your query.
The description that you provide suggests some blunt injury
to the area which has give rise to the lump and the overlying bruise
. I suggest you can wait for a day or two during which time the lump should reduce in size. However, if it persists then it will be advisable to consult a doctor. In the interim, you can apply warm fomentation over the lump.
I hope I have responded to your query to your satisfaction.
Wishing your son an early recovery.