That's exactly why he can use Li ion without a problem. Those are not for the average Joe. You have to understand the ins and outs and do your own research before using them. Don't worry, it's not like playing with radioactive stuff but they still require some precautions because they are POTENTIALLY dangerous. And yes, you need a dedicated charger and no, they won't fit in every flashlights. You better stick with AA (LSD rechargeable like Eneloop) for now.