I have only played 5E two or three times, so forgive my total newb-ness if this question seems off-base. My area of expertise is in the first 3 editions of the game. I don't have anything against the 4th and 5th editions, it's just that I stopped wanting to re-learn the game / buy new books past a certain point.
Can't he simply create a unique spell, that allows him to charm people for a given time, without concentration? I know the rules probably say something like "not having to concentrate on the spell would be unfair / unbalanced," but think of it like this:
If this were my campaign, I'd let the NPC have developed such a spell. And it would have had the following drawback: every so often, he'd need to make a Willpower save, and if he failed, he could either allow the spell to end, or keep it going by spending "brain cells" (and I would then make up some arbitrary system to explain what brain cells were, and how many a given character had). Thus, the explanation would become simple: Excapode kept spending brain cells to keep his spell going, until he ran out, and went quite literally insane.
In the spirit of "what the NPC can have, the player should also be able to have," at the completion of the adventure, the PCs could find the spell in his spell book, and then they would face a dilemma: use the spell for themselves, and possibly have one or more party members go insane, or rip that page out and destroy it, and gain xp for destroying an evil item.
Done.
Please, don't applaud. Just send money. Ha ha.
