Hi M.---It sounds like you are pretty much on your way if your little one is drinking water from the sippy. Maybe start with putting formula in the sippy cupy for a while. Really, all he needs to drink is water when transitioning from formula to regular table food. I do NOT recommend even bothering with cows milk. It is a highly allergenic food for humans, meant for baby cows to grow into big cows in a short period of time.
I apologize as this does not answer your question directly, but this is such an important subject that is not well known, I am compelled to offer this information so that you might make the best decision possible for your family. I offer this thought as a reformed milk drinker and I've learned about the dangers of cows milk from a series of wellness classes I am taking, taught by a Naturopath who has her PhD in Nutrition. Dr. Pam shares advice based on long term research and I would be happy to get you copies of articles that she shares with us.
In the meantime, you can explore www.pcrm.org and www.strongbones.org. Another website is www.notmilk.com and, from Dr. Weil, http://www.drweil.com/drw/u/id/QAA182741.
Edit: You might find it interesting to know that the advice to drink milk for bone health comes from milk producers and from the USDA, which was started to promote farm produce. So we really are getting biased info from these sources. Then, our physicians basically don't know what they don't know and tell us that milk is good because that is what they are told. Milk actually increases the risk of type 1 diabetes in kids and the risk of prostate cancer in men. Skim milk increases that risk for prostate cancer even more, as it concentrates the milk proteins, which are the problem. I also recommend reading The China Study by T Colin Campbell.
Don't worry about calcium. We get more absorbable calcium from green leafy veggies, such as broccoli, bok choy and kale than from milk. Other great sources are sesame seeds, fortified OJ and plant milks and others. Feel free to contact me if you would like more info on this subject. Good luck, be well. D.