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.不考虑句义，只看I will tell you that he said.是否是正确的句子不行。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。