He's on board, though I'm not sure I like the idea of having his character just wake up kidnapped, that seems to much like railroading to me.
Once again, it's not railroading if the player is in on it and cooperating. Seriously, have you never hatched a plot, as a gm or a player, where something happens that you both want, without rolling some dice"?
Some years ago, one of my players, a very tactically minded, great roleplayer, went kind of crazy. He had this great plan to bait a monster, and then follow it to it's lair to easily kill it and rid the kingdom of it's terror. In fact, he went to great lengths to get a promise from the loose cannon of the group to follow his lead. Then, in a momentary lapse, just as it was about to leave, he attacked it. Hilarity ensued.
Afterwords, he had no explanation for me and seemed as surprised as I was. He concocted a scenario where he was replaced by a doppelganger some weeks before and the next few sessions were about the 'real' him finding the group and outing the imposter.
The point is, the whole replacement thing could have been seen as railroady, had it been done without his cooperation (though I have done that as well). With his cooperation, however, it just becomes part of the weaved narrative between the player and gm. And that's...a good thing.