Then I would just rinse him off with water and pat him dry. Did you call the vet? they will give you some suggestions without charge. I hope he feels better soon.
I'm not quite sure how he got it. I would think that soap would irritate it. There are no other reactions, but he has really sensitive skin anyway.
I know you'll probably want to put on some gloves and bathe him. Rinse him with water to flush it away. No soap, I would think that would irritate it? You might want to call the vet and ask what they recommend. Did he eat it? Is he having any reactions (other than his skin) to it, vomiting, diarhea??