I have no problem with trusting DS (13) as far as behaviour and following rules are concerned, I do, however, worry about his handling of a wave phone. I am terrified he will leave it somewhere, or lose it and we will be stuck with the charge!!!! He is likely to be so engrossed in an activity at Edge or somewhere, put it down and forget to pick it up. I realise he could attach it to himself with a lanyard or a clip, but it is easy to detach it, say, in the bathroom, and forget it.
I understand there are phones everywhere (in the hallways, kid's clubs etc.), you can use to call wave phones. Are they abundant? Could he use these to keep us updated with his whereabouts and his plans?
What have other parents done?
My daughter, 10yo at the time, was fully aware of the cost of the phone so she was extremely careful to keep it on her at all times. But that's also her nature to a certain extent. Would your son be opposed to a fanny pack? Yes, they're lame, but they'd be perfect for keeping the phone in.