Re: Two questions
1. No need to use cow's milk unless you want to. Your LO can get all his needs for "dairy" met by you, provided you nurse at least 3-5 times a day, or by whole milk cheeses and yogurts. That being said, offering cow's milk in a bottle or sippy doesn't seem to have much impact on duration of nursing, since it is much, much less delicious than breastmilk and comes in a much less interesting container. If you do decide to use animal milk, it should be introduced only once baby is >1 year.
2. Don't worry about it. Just keep offering the sippy. Your LO will get the hang of it, probably witching the next few months. And while weaning from the bottle "by a year" is recommended, success at getting kids off the bottle varies a lot. If your LO still has a couple bottles per day at 12 months, or 18 months, or 2 years... Trust me, he won't be the only one!!!
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"