What's the matter with him?
身体不好的话可以回答：
He's ill /cold .
He's got a high fever /a bad cough.
具体其他什么原因也可以直接说明：
He failed the exam./He was late for class.
没什么事的话：
Nothing.He's fine.
不知道的话.
I have no idea./I don't know.