What Causes Fluid Filled Lumps At The Back Of Head?

16 year old son has 2 large fluid lumps back of head,,felt first lump xmas time thought it was a boil,, then forgot all about it as nothing caused any concerns until my son asked me to feel his head during Sunday evening just gone as the lump I'd originally felt during xmas time had grown evidently not a boil like originally thought,i have just arrived home from work an my son asked me to feel his head again as another fluid type lump has appeared now quite concerned?