你怎么会不知道他在说什么
How is that you don't know what he is （was） talking about?
How come you don't know what he is （was）talking about?
How could you don't know what he is （was）talking about?
或者
You don't know what he is (was) talking about?How is that?
You don't know what he is (was) talking about?How come?
You don't know what he is (was) talking about?How could you?
其中don't know可以换做 have no idea是一个意思
"他在说什么"指的是现在,用进行时态比较准确,即使是过去说的,也应该是刚刚发生的事情,也用过去进行时比较好.