Rash?

I need help with my 9 month old's diaper rash. He's had diarrhea for a day or 2 and it's caused his butt to be very red and raw. I've tried cornstarch, baking soda baths, butt paste, a&d, desitin, letting it air dry. I've done everything that I can think of but it isn't getting any better. He screams every time I touch it with anything. Does anyone have any suggestions