I would try to stop for a while. Mean while, have you thought of getting him a new bottle. Maybe you want to bring him along to choose? In that way he can see that its new. Other alternative would be to ask him to join you in washing the bottles. Explanation and visually seeing does help a fair bit.
Hi, I think now that your baby understands and associates the smell with the bottle, I think you can try getting a new bottle for her. Better, try feeding her from a small cup, and if she is comfortable with it, you will not have to be stuck with bottles anymore.
Try spoon feeding him the freshly expressed breastmilk. Dip the nipple of the bottle with the freshly expressed milk. So he would be able to smell that it's not the smelly milk.