Welcome to Healthcare magic forum.
Considering the details you have provided your son most probably has a pustule due to a bacterial infection of the skin.Such infections are common in children and most resolve within a week with minor supportive measures like cleaning, dressing and local antibiotic
ointments like neosporin
, or fusidic acid
However, if the child develops fever , increase in the size and number of pustules, worsening pain or swelling of the affected limb, then it's best to consult his pediatrician
for confirmation of diagnosis and starting apropriate oral antibiotics
As of now the situation doesn't seem worrisome and you may wait and watch for resolution of his symptoms for a couple of days .If he develops any new symptoms or worsening of the lesions then please seek prompt consultation.
Hope I have answered your query.
Wishing your son a speedy recovery.