welcome to healthcare magic forum
thank you for your query
i understand your concern regarding your son's skin problem.
there is nothing to worry about it.
it is just simple bruise
develop after hitting.
it will gradually fade out in 5-7 days.
you can apply antibiotic cream over it.
he might develop small swelling over it
if he develop pain ,fever
or other severe symptoms,it is better to consult doctor
i hope i answered your query