I don't know enough to judge!
DS3 really whacks his head when angry (which is frankly all the time he has a vile temper!). He hits it hard enough to bruise (has two bruises at the moment- one on the forehead and one on his nose from belting his face into a trolley). I know he's worse when he's had certain foods (esp gluten, cows milk).
We're seeing a chiropractor this afternoon so will get him to check he hasn;t got a squashed bit or anything. He may be copying ds1 to an extent, as ds1 hits his head a lot, and sometimes ds3 will hit his head straight after seeing ds1 do it.
Getting cross with him/ignoring him when he does it does seem to work a bit. I must point out he is hitting himself hard enough to hurt himself though so we can't completely ignore it.