Cause For Shape Of The Head To Change In A Child?

my 3 year old son has been healthy since he was born. But within the past couple of months the shape of his head is changing. It has gotten flatter on top (block-head, or almost frankenstein in appearance), and I noticed last month a dent/dip along the coronal suture. This dent is new -- it has not always been there! It is not like a ridge or a bump -- it is like a 1-inch wide canal -- as if he had worn a headband for years and his head formed to it. It isn't very deep, and isn't really noticeable through is thick hair. But it is definitely palpable -- we've had doctors confirm it is there, and they aren't sure why it would suddenly appear, but they had x-rays done that came back normal. What's next? What could cause this in an otherwise healthy pre-school aged child?