C.M.
Tell him it's time to give the bottles to the new babies. Make a big deal out of putting them in a box and "mailing" them. Then get rid of them. Trust me, when he is thirsty, he'll drink. Also, I would strongly suggest replacing the bottle part of winding down with something much more attractive. A special songtime, a backrub, a new soft and awesome blankie, a special new book. He will be less likely to protest the loss of bottle if you have something he wants more.
Good luck! It won't last too long.