Could you tell me what's the matter with him?

what引导一个宾语从句 what's the matter with him,原本就是陈述句语序.如：He told me what's the matter with him.
what是主句的宾语成分,同时引导一个定语从句,是定语的主语.what=it that
What do you think is the fastest way to travel?中what引导一个主语从句What do you think. what是主语从句的宾语成分,同时是主句的主语.这里也一样what=it that.陈述句语序是：I think what(=it that) is the fastest way to travel.