Try just giving him a bottle (or cup) of milk. If he likes it, then switch one bottle a day to milk, and after a few days switch another bottle to milk, etc., until he's completely switched over.
If he doesn't like the taste of the cow's milk, then you can try mixing the formula with the milk, gradually using more milk than formula. Start out with 1/4 milk to 3/4 formula in all bottles, and after a few days up it to half and half, etc., until he's completely switched over.
I don't remember noticing any change in poop when my son switched. And something else that helped us was using some chocolate flavoring to get him to drink the milk; not a lot, just enough to make it a bit more appealing, and we used the sugar-free Nesquik powder. After a couple of times like that, he went to straight milk with no problems, and he still loves milk at 2 years and only gets chocolate occasionally as a treat. Something to consider. :)
at 9:37 PM on Oct. 10, 2009