"He could hardly imagine that Okonkwo was not his real father. He had never been fond of his real father, and at the end of three years he had become very distant indeed."

The great bond between  Ikemefuna and onkonkwo developed in ekemefuna a great affection towards okonkow . even though okonkwo is not his real father ikemefuna sees him as if he were.  Therefor he shows no fear as he is being taken into the woods to get killed.