How to describe a person who have a fossilized mind?
whatever he hears he will not (get it into one's head)
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You may be thinking of "thickheaded":
thickheaded — (of a person) dull-witted; stupid
The connotation is one of stupidity or inability to understand something:
Alice is so thickheaded she couldn't understand the instructions you sent to her.
Bob is pretty thickheaded so you may need to use small words.