Most likely, it's a complication from the actual circumcision itself. Infections are the most common complication seen in circumcisions. Any cut or wound can result in infection. The bacteria Staph. aureus is all over the skin and especially rampant in hospitals, and can easily infectect a wound, causing a staph infection. Since babies are always in diapers, the groin is moist and bacteria thrive there. A big concern here would be whether the infection is a normal staph infection, or something worse called MRSA (methicillin resistant staph aureus) which is resistant to a lot of antibiotics, harder to treat, and much more serious. The culture they did will be able to determine this.
Staph infections can be very serious if not treated properly, so it shouldn't be taXXXXX XXXXXghtly. MRSA is even a potentially fatal strain. Your son should be on appropriate antibiotics to clear this up (the culture will let the doctors know which medications are appropriate), and do keep the area as clean and dry as possible. You may want to wear gloves while treating the area, as MRSA is highly contagious. Be sure to wash your hands with a disinfectant soap after touching the area, and do not touch any other part of your skin or the baby's until your hands are clean to prevent spreading it.
You should follow your doctor's instructions for proper cleaning care, but it usually involves washing with warm, soapy water, rinsing thoroughly, making sure the area is dry, and applying an antibiotic ointment if inidicated. Frequent changing of diapers and powdering may help keep down the moisture level (which bacteria love to grow in).