Here's why:
* "Whom" is the object pronoun used for people. It's the version used when the pronoun is receiving the action of the verb.
* "Meet" is the verb in the sentence. The action of meeting is being done to someone.
* "Jimmy" is the subject of the sentence. He is the one performing the action of meeting.
Therefore, "Whom" is the person being met, making it the object of the verb.