My 12 yr old son has a lymph node in his neck that has been swelling & shrinking for 4 mos. his dr said it was allergies & wasn't worried.sound right?

Common: Lymph nodes are part of our system to help fight and prevent infection. It is not uncommon for them to get a little bigger and recede from time to time. I don't think i would worry by what you are describing.

How large?: If they are buck shot size, then it just needs to be followed. If it is a pecan size one, then i would be more concerned. Absent other symptoms, i would make sure that he does not have sinus or dental infection. Allergy is not usually associated with noticeable node swelling.