Ask him what he had said.这句话有语法错误吧?

是的 Ask him what he said.

ask him what he said好些
原句也没有错，但不自然。

要看具体语境
这样但拿出这句话不能说明什么
如果参考点是过去的某一点，那就没有语法错误
例如：A: I hear that Jim was fired.
B: Yes, ask what he had said to the manager.

Ask 后面不能用had``
要保持前后时态一致``
Ask him what he has said.

Ask him what he said.