What Causes A Lump In Front Of An Ear?

hi, my son has a lump infront of his ear and it is hard and it stops him from eating, we have had him tested for mumps, it wasnt that and had him to the dentist, they told us it was his salivary gland, so they told us when it happens to rub the inside of his mouth to get the saliva moving (sry for my spelling) that doesnt help, this has ben going on now for the past 3yrs on and off, what is it and what should we do? i wanna know and am doing as much research on this as i can