The bottle attachment is a tough thing. I know it sounds mean, but I would take away the bottle cold turkey. Expect a lot of crying, but if you can put up with it, it only takes three days. I use this method to get rid of bad habits. If my child got up in the middle of the night, I would check to see if he was okay and dry, and then I'd let him cry it out. After three days of him crying for 45min in the middle of the night, he slept through the night and has ever since. If your child is in a safe environment and cared for, a little crying is not going to hurt them. If he gets hungry enough or wants the milk bad enough, he'll drink from the sippy cup. You just have to be strong and not give in. Just a tip, if you have any other children, try switching to a sippy cup when you switch to milk. New cup, new drink, it works!
P.S. If he is a bad teether, try those teething rings you stick in the freezer or rifrigerator. They are usually made from the same stuff nipples are. So, it may be a good alternative if he needs to bite on something.