I will tell you all that he said .all that可否换成that

语法没问题.这里 that 可以省略,读起来会更顺.
I will tell you all that he said
我会告诉你他说过的所有话.
不行,因为直接宾语 all 是先行词,后面的定语从句修饰all.
可以改成 everything 之类.
再问： 不考虑句义，只看I will tell you that he said. 是否是正确的句子
再答： 不行。可以改成：I will tell you what he said. what引导宾语从句，相当于 the thing that，包括先行词和连词。