简单的英语问题。懂的进来

因为By then 的存在所以句子中要有完成时态，by then又是过去时态，he knew发生在过去，he wanted to be发生在he knew之前，是过去的过去，而gorw up 又是客观事实所以不随时态改变应为现在时，所以句子应为：By then, he knew what he had wanted to be when he grows up.

不对吧
如果情景是在过去的话，应该是By then,he would know what he wanted to be when he grew up.
如果情景在现在的话，应该是By then,he will know what he wants to be when he grows up.