You aren't doing anything wrong if your son is happier and has had no adverse reactions to the cows milk then it's far better than a constipated baby. I had the same probs changing DS from breast to formula - two weeks of not much rest and alot of stress and early morning calls to the health nurse number. He went 7 days without a bm. Never thought I'd be so happy enough to call DH at work to tell him we had movement at the station, haha.

I didn't introduce cows milk till my DS was 11mths and it was for no reason really other than I felt like changing him over by that stage.

You're doing a great job and I can tell you have done everything to make your son is comfortable. 8-)