Thanks for posting your query.
Its most likely that the antibiotic he is being given is not effective in clearing the infection as the bacteria causing the impetigo
might be resistant to that antibiotic.
I would recommend that you visit your doctor and request him to send the pus or fluid from the lesions for antibiotic sensitivity test.
Also its important to know what organism is causing the impetigo and whether Tyre correct antibiotic is being administered.