Instead of requirements for activation, maybe he can only use it so many times a day. 2-3 times or smth? Or maybe each time he uses it, it takes a chunk of energy and he needs to recover it. The more damage he takes /the more people he helps, the more energy he uses.

I also support rolling 1d6 for fun too, (but I think in moments of cool rp moments, its ok to just say it works and such if that makes sense).