There are so many possible causes of such a swelling, most of them quite benign, that it is impossible to do more than hazard a guess from a distance this way. However, it sounds very much like a swollen parotid gland, one of the lymph glands which lies directly in the carotid chanel beneath each ear. These often will swell (as will other lymph glands) because of a transient infection. Sometimes one is also just naturally larger than the complimentary one. Still, especially in the case of small children (who cannot offer their input) it is wise to have it checked by a doctor. While there are potentially more serious causes for something like this as well, if your son doesn't seem ill then it probably isn't a serious issue, but again, since the child can't offer much information (other than exhibiting a fever or other signs of infection) it's best to have the doctor look at him. Hopefully by now that's been done and all is well.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist