My 6-yo son has a small hard immovable painless bone-like bump behind his ear. ?

Likely lymph node: The area behind the ear is called the mastoid bone. Often there is a tiny lymph node over this area that can appear quite prominent. As long as it is about the size of a pea and mobile, it is normal. Next time you seen your doctor, have it checked. All the best.

Mastoid bone or node: The mastoid bone is the part of the skull behind the ear, and sometimes it is irregularly shaped, but would be naturally immovable and painless. Alternatively, there are lymph nodes behind the ear that swell at times in response to minor inflammation in the skin around the ear or ear canal that are usually quite firm as well. Unless it continues to enlarge or become painful, don't worry.