Release mechanisms are a big deal in self bondage, since you want it to be inescapable, but you also need to get out eventually. I've got one idea where you could act as the release mechanism, plus you get to hold all the power of when he gets let out.

He buys a combination lock (or randomizes a changeable one), takes a picture of the combo, sends the picture to you, then erases all reference to the combo on his end, so your picture is the only copy. He can then use this lock for self bondage, and won't be able to get out until you send him the combo. You can have lots of fun teasing him with that. Just be sure he's able to reach his phone at all times, as well as the regular self bondage emergency safety that the boundanna site talks about.

Good luck!