H.,
My son had multiple digestive issues his first 2-3 years, including constipation. For a quick fix, give him half a Baby Fleet enema (did this on doctor's advice), or 1/4 of an adult one if you cannot find the baby ones. They just have glycerin in them. You might need your husband to hold you son still so that you can do this. Our son was not a fan! Then get a diaper on him quick because this stuff works! I actually used one on myself first (yes, ewwww!) because I wanted to know what he would go through. I had about 30 seconds to a minute of cramping (uncomfortable but not super painful) and then, well, success. Our son appeared to experience the same thing. Once you have the immediate situation taken care of, you might want to wait another month on the milk. You are smart to make this switch gradually (just 1/4 of the bottle for about a week, then half, etc.). You can also consider using prune juice (though we needed to go right to stronger stuff) or Milk of Magnesia (ask pediatrician for how much -- we used to put it in a syringe, no needle of course, and put it straight into the back of his mouth) to help him when you start adding the milk again. Once he has been on milk for awhile (assuming the MoM works for him), you can start decreasing the MoM. Again, please check w/ your pediatrician first re: MoM. Also, don't let people convince you that your son is lactose intolerant -- highly unusual at this age, and breastmilk has lactose in it. Both my husband and I are lactose intolerant (maldigesters, actually, because we can have some dairy), but that was not the issue with our son, and I was sure it was! This also does not mean your son is allergic to milk. Additionally, don't jump right to soy. We tried soy and it was just as tough for our son to digest. Also, allergies to soy are pretty much as common as milk allergies, and soy has compounds in it similar to estrogen, which is why so many menopausal supplements have soy. The medical community does not have consensus at the moment around the effects of young children having a large amount of something that acts like a female hormone. In countries where soy is a staple, it is often fermented, which changes the chemical composition in some way I don't understand (not a chemist!). I hope some of this helps.
K.