He needs to be kept in line.
keep sb. in line直译过来就是：“让某人排队”，这个短语的正确意思是：“管束，守规矩，让某人老实点”。因此，当美国人说"He needs to be kept in line."时，他/她要表达的意思就是:"He needs to discipline himself."、"He souldbe taught to behave himself."。
Benjamin: My assistant always does what he thinksright, and he never cares about others.
Boss: He is unaware of his behaviour. He needs to be kept in line.
①I agree with you. Like teacher, like student.
②I'm mad at him.
③He is going too far!
④Otherwise, he will cause trouble sooner or later.
discipline vt. 自律
Order and discipline have been placed in the hands of headmasters and governing bodies.
It was that image of calm and discipline that appealed to voters.
True discipline is self-discipline.