Why does my 10 month old always have a bacterial infection ?

For the last two months my now 10 month old son has been having issues with his right side of his face. It first looked like a rash. Then it didn't go away. So we went to the dermatologist she said he had eczema and took a culture of his face. 5 days later it came back as a bacterial infection. She called in an antibiotic to help to go away. It went away. Then three weeks later he gets it again. Bacterial infection was cultured again and it came positive again. He was on another antibiotic. Thought it was gone. Now it came back 6 different times. 6 different antibiotics and it goes away and comes back. Cultures say there is none after the infections gone. Why does it keep coming back?