Inb4 he gets a cheat item that lets him store MP, but in some circumstances, constantly drains it from the wearer to fill its storage instead. Probably intended as a means to keep prisoners from being able to use magic, and instead turning them into sorta MP battery chargers. Nobles might have those kinds of rings as backups, and he gets one, has it drain MP from him, and it fills up instantly. Then he's got a billion MP or something crazy like that at his disposal, and "oh, it ran out? Okay, drain me. And... full. Another billion MP ready to go!"
I'm thinking, like, a ring which, worn one way, drains MP as fast as the wearer recovers it, worn the other way around, lets the wearer use the MP it's drained and stored. To visualize it, it's a two banded ring, one half black, one half white. White side being closer towards the base of the finger gives MP, black side closer drains it. Or a bracelet. Something easy to reverse like that. Could even be a "on the left hand, does one, right hand, does the other" and just have one (or more) on each hand at all times.