My son has a hard fixed lump on back of head more to left side parallel with top of his ear. No injury but does have dermatitis on right side of scalp. Seems too high to be a lymph node. But note sure. What could this be?

? mastoid process. From your description and location, this sounds like the mastoid process, which would also be present on the other side.This is a projection of the skull where many of the jaw muscles are anchored. There are a variety of lymph nodes located all over the head and neck, but these tend not to be hard. Your doc could confirm or exclude this by exam.