问个英语句子,过去式和过去完成时的辨别.

问个英语句子,过去式和过去完成时的辨别.
老人与海的开头：He was an old man who fished alone in a skiff in the Gulf Stream and he had gone eighty-four days now without taking a fish.In the first forty days a boy had been with him.
最后一句 In the first forty days a boy had been with him.
语法书上说过去完成时是过去的过去,
可是这个句子in the first forty days不是过去的时间吗?
为什么不是a boy was with him?哪里体现了过去的过去?