What is a lump on the back of the head?

Dandruff can cause swollen lymph nodes

I've searched this for weeks because my 12 year old son has the same thing in the occipital region of his scalp. What I've read is that dandruff or seborrhea, ringworm or a bug bite can cause the lymph nodes in that area to swell. At age 12 ish, the lymph nodes are the largest they'll ever be so any swelling is more noticeable. The lymph nodes on the back of the head almost always swell because of scalp infections, pimples, cysts, infected hairs, bug bites on the scalp and almost never, ever from cancer. Those nodes collect gunk from the lymph system of the scalp and not deep within the body.

There are 1 - 3 nodes in the area behind the ear, in the hair area and on both sides of the head. My son has one that has remained swollen for four weeks. Depending on the size, the nodes can take 4 - 12 weeks to go down AFTER the infection / problem has been cleared up.