What Causes Regular And Recurrent Episodes Of Pus Boils On Scalp?

My son had head mundan 1 year back, after that he is having regular recurrent episode of Pus boils. Dr have already done many treatments including antibiotic, fungal treatment. All tests including Immunity check also came normal. Now for last 6 months, i am using Ketagold shampoo daily, but still pus boils come and go every 1 month. Although i don t apply anything on them now (no ointment) and no medicine. They get clear in almost 10 days everytime by themself. But then again come in 20-30 days. I am keeping all hygienic precautions, no oil on head, but still this is going on. Please suggest.