Hi D.,
I know you already got tons of responses, but I thought I'd add my own as a different angle...
I personally don't think that cow milk is necessary for kids, and many nutritionists agree. If your little one gets a good variety of food, he/she will get enough calcium. Or I suggest using the formula made for kids 9mo-2 years to get some of the added nutrients. We only use cow milk because 1) they produce more milk than smaller mammals, and 2) are easier to domesticate than other large mammals. Nutritionally, it's not even the correct balance of fat, enzymes, etc. for human babies to act as a replacement for breastmilk, so really, it's over-used as a meal-replacement. Again, this is just my opinion (and the opinion of many professionals), but there's a lot of evidence supporting it, and at least it offers a different perspective.
If your son is under weight, then whole milk would serve a purpose, helping him to get the fat content he needs, but otherwise, it's just not necessary. I nursed my son for 6 months, then did formula for almost 6 months, with solids being incorporated. He's perfectly healthy, the right size, energetic and intelligent, and has never had milk as a drink. The only time he has it is when it's part of another food, i.e. cheese, yogurt, used as an ingredient in a casserole... He hasn't suffered in any way by omitting cow milk as a nutritional supplement. If he had it and enjoyed it, then I would give it to him as a drink occasionally, but only because it was something he liked, much like I like lemonade, even though it doesn't provide much other than vitamin C.
If you stick with cow milk, I suggest trying an organic milk, not only because of the normal reasons (no hormones, chemicals, etc.), but because it lasts longer in the fridge! I couldn't believe it; I had a container of organic milk for cooking for me and the kids, and my husband bought a gallon of regular milk because he doesn't care what he drinks, and even though they were purchased within a day or two of eachother, the expiration date on the organic milk was around 2 weeks later! In the long run, if you don't drink tons of milk, you're getting a better value by buying the milk that lasts longer, so you don't have to replace it as often.
Good luck in transitioning, no matter which route you take!