D.M.
Hi N.---I would give him just his formula and then transition him to drinking water for a beverage. Cows milk is not meant for humans. It is formulated to make a baby cow into a big cow in a matter of weeks. Cows milk is highly allergenic and is a leading cause of type 1 diabetes. It also contains a compound called insulin growth factor, which increases the risk of some cancers.
I say all of this as a reformed milk drinker. I have learned, taking a series of wellness classes taught by a Naturopath who has her PhD in Nutrition. Her advice is based on long-term research which is quite amazing considering all of the advice stating milk is an essential part of our diets. But, please don't take my word for it. Do some research starting with www.pcrm.org and within that, www.strongbones.org. There is also another site called www.notmilk.com.
Having said that, I will, on occasion, use half-n-half or some cheese, organic, when a special recipe absolutely requires it and I wouldn't want to substitute an alternative milk product.
Don't worry about calcium. There are many other more readily and easily absorbed sources, coming from green veggies, particularly broccoli, kale and bok choy. Other good sources include sesame seeds, and tahini, which is a component of hummus. Fortified OJ and other plant milks also contain a good amount of calcium. If you'd like a more complete list of calcium rich foods, please feel free to contact me. Be sure to get a decent amount of unfiltered sunlight to help the body make adequate vitamin D which is essential for calcium absorption.
Hope this helps. Good luck and be well. D.