What Causes A Lump With Dark Foul Discharge?

my boyfriend had a large lump on his back for at least two ears. it was somewhat dark looking. finally after two years one day it smelled cheesy and when i squeezed it an unbelieveable amount of dark smelly puss came out.... i thought it was gone. it has been two months and every few days i can squeeze it and dark puss still comes out.??? what could this be