This is a good short story that could be a really great one. You've got the idea, and damn if it isn't worthy of the greats, but your treatment needs some cleaning up.

I'd make the transitions between times a little smoother -- him with the fat man, then him eighteen months ago, then him with the fat man again, et cetera. I'd also give a little more detail about what it's actually like to be in someone else's head, whether there's anything special about it, if he was amused by his wife's default thought processes, something like that. I'd also clean up the ending a little -- far from being confusing, it's a little too concise and you don't need to lay his plans out for all to see. After you've got the story, it's pretty easy to figure out his plans. Basically, what the story needs is just a little beefing up, more descriptions and things like that.

But, as I said, your story is really cool and I can tell that you have a great deal of literary talent already. This story just needs a little bit of tweaking and it'll be incredible. Good work!