Bsd diaper rash

So my LO has a bad rash. It was just red at first, so I used a butt paste. Now it's started bumping up and his little balls are super red. So I just did a little baking soda bath and ran without a diaper. He hasn't had anything new here, but at my in-laws this weekend he tried soy milk. Why? I have no idea. He does fine with whole milk. Could it be that having a later reaction? We change him as soon as we see his wet diaper and put powder on it to keep it dry. I would think if it was the soy milk that it would be getting better not worse. Any advice??? Help!!