My Son Has A Purplish Lump Behind His Ear Lobe,

my son has a purplish lump behind his ear lobe, said it hurts and itchy at tims. he has had it on and off for 3 months, it may go down but not away, went to dr. did sonogram couldnt see anything. it is behind the ear next to ear lobe like right in the crease between the earlobe and head. more oblong shaped than a circle. any suggestions of what it might be?