It is possible that the lump could be a possible hematoma
in the muscle following the injury
. It seems benign as for now. Please wait and watch for another week. If it subsides on its own, then nothing to worry.
I do not think it is very serious. If it remains after a week or ten days, you may get him examined by the pediatrician
to look for a possible inguinal hernia
or a lymph node.
I wish him quick recovery.