You're not alone. My 16-year-old son started blocking up as soon as we put him on milk when he was a baby, and still cannot take most dairy products. He would end up in the hospital with intestinal blockages every time we tried milk; we know other families where the kids don't block entirely, but have major bloating problems. We've narrowed down Jordan's intolerance to the milk protein, casein, which can turn into glue (this is no exaggeration -- you can actually paste things together with it) when it's treated with vinegar (acid), heated, and dried out on filter paper. We know that cf makes the intestine more acidic than normal, and it's both warm in there and tends to be on the dehydrated side, as the basic defect in salt transport draws too much water away from the gut contents into the cells. Great conditions for turning any milk protein passing through into glue.
So one thing you might try, if you haven't already, is to give your child antacids to couteract the extra acidity in his intestines (this makes the pancreatic enzymes work better, too). Also, he could take lactulose syrup, which helps pull water back into the intestinal contents, reducing constipation. And prunes do help because of their natural laxative property. For us, though, it was necessary to avoid all casein-containing milk products except those that are cultured (like yogurt), where the protein is already partially digested by the culture bacteria. Butter is OK, as well, because it's mostly fat, rather than protein. Fortunately, there are now lots of substitutes for milk products on the market (only be wary of soy cheeses, which usually contain casein), so your child doesn't have to feel deprived, even with no milk at all.
Let me know if I can help in any way.
Bambi, mom of Jordan, 16 w cf