Diaper rash after weening from breastmilk

I just stopped nursing my son now that he is 1. I had tried introducing organic whole milk (cow) and he immediately developed a diaper rash (which he had never had before ever). I assumed it was the milk and discontinued it. A few weeks later I gave him some organic yogurt made from whole cows milk, and again, major diaper rash. Then I tried goats milk and everything was fine for about four days. So I figured all was good and stopped nursing him. Well, now the diaper rash is back. What do I do? I can't go back to nursing (milk dried up). The goats milk is the only new food introduced in his diet. He is not constipated, in fact he kind of has diarrhea.

Any suggestions would be so appreciated!!
Juli