What Could A Lump Behind The Ear In A Child Be?

hi My 15month old son has had a palpable, non tender lump behind his right ear for 6+ months. Our GP sent him for an ultra sound in November 2009 and the findings were it corresponds to a small lymph node ...normal oval contour and maintained fatty hilum and is non enlarged The GP thought no further analysis was required. There haven t been any blood tests done. The lump has remained approximately the same size. I would like another opinion on this. regards, Sandra