Find my iPhone is gone

My son has an iphone 4. Not an "S". We enabled find my iphone so I can keep tabs. He's aware of it and it was part of the agreement to get this phone. But last week when I attempt to locate him on my phone (a 4S) the rest of the house iphones come up but his says "no location available." He and I looked at his phone and went in to settings and cant find the "find my iphone" prompt anywhere. He says he did nothing to disable it, and we cant figure how to enable it again. Help? Thanks