How to recover my son's stolen iPhone?

My son's iPhone was stolen and he will not help me locate it. He says that he was able to see it at the front of his school -- I think the person who stole it probably hid it under a bush until the battery runs out. I would like to try to retrieve it, but my son refuses to help me use the Find My Phone App. What can I do to try to recover the phone?

[Re-Titled by Moderator]