I would remind the student of the boundaries of the plan and remain consistent with your expectations. Do not reward his manipulation with attention but rather redirect the behavior towards a more appropriate one. If misbehavior continues I would set up a time to discuss a new plan of action with the students parents and possibly the guidance counselor depending on the behavior.

I am a little confused on what you mean by taking advantage, but I would talk to the student and figure out why he is misbehaving. Talk to the parents again so they know about their child's behavior and could help set a new plan in place.