Kind of already made the mistake. You didn't say that he actually sucks on it during the night. I take it that he just holds on to the bottle? That he doesn't treat it as a paci?
I would actually stop rocking him. Give him the bottle and put him to bed, groggily awake. Not asleep. Make sure he has burped. Leave the empty bottle in there. If you see him with it in his mouth while he's asleep, carefully pull it out. If he gets to the point that he forgets to put it in his mouth, but sees it "there" when he wakes, I think that's okay. It's a little like a lovey.
When you wean him off a day bottle and onto a sippy, put an empty sippy in the bottle's place. He may cry like the dickens, but NO BOTTLE. He'll get tired of the sippy and sleep all night if you DON'T go in and pick him up and let him know that you'll come in and BE the sippy/bottle substitute for him.
Good luck,
Dawn