This is such a long thread, so sorry if this is old news... I didn't have time to read everything.
Just wanted to say quickly -- we were in the same boat and I was able to quickly find the culprit (took a few months) in his newest favorite food -- chocolate milk. DS never had any cow's milks until about age 3, and then he discovered choc. milk and also had the major constipation and potty fears.
We stopped the cow's milks -- it ended within about 48 hours. A kept him milk free for several weeks, happy pooper. Let him have some milk... BAM! Plugged up again.
If you google "milk allergy constipation" you'll find lots of info...
Here is a quick quote from a study backing this up:Researchers at the University of Palermo in Italy worked with 65 children with chronic constipation. All of these children had been treated with laxatives when dietary measures had failed. Even with the medical treatment, these children were still constipated, having hard, painful stools only every 3 to 15 days. Forty-nine of the their little bottoms had fissures and redness or swelling from the hard plugs of stool.
Each child received either cow's milk or soymilk for 2 weeks, with no one knowing which was which. Next, they had a week during which they could eat and drink anything they wanted to wash out the effects of the first 2 weeks. Then they switched sides for 2 weeks and got the milk that they didn't get the first time. Careful recordings of the bowel habits were made.
When the secret code was broken at the end of the study, they found status quo constipation for each child while he or she was on cow's milk. But while they were taking soymilk (which causes firmer stools in most kids), 68% of these kids were no longer constipated! The redness, swelling, and fissures on their bottoms healed (New England Journal of Medicine, 1998; 339:1100-1104). How wonderful to finally have relief after diet and medicines hadn't worked for so long!
The results were most dramatic in kids who also had frequent runny noses, eczema, or wheezing. Nevertheless, sometimes constipation can be the only symptom of cow's milk intolerance.
This has broad implications. The children in this study were those with severe chronic constipation that was unresponsive to medications. I am convinced that they are only the tip of the iceberg. There must be a much larger group of mildly allergic children whose constipation improves with laxatives. Time may prove that it is better for these children to avoid the offending protein by switching milks rather than being treated with laxatives.
Presumably, swelling of the intestinal lining causes the constipation. Whatever the exact mechanism, the problem is with the protein in cow's milk, not with the fat or lactose (the sugar). Skim milk or lactose-free milk will not help with this one. Switching to soymilk and other soy products might transform the life of your son in only a couple of weeks!