Hard to avoid diaper rash if the diarrhea doesn't stop. When they have diarrhea, my doc says to avoid all fruit, veggies and dairy. Feed rice, pasta, potatoes.. lots of bland carbs. To drink, if your son won't take plain water, my doc recommends diluted Crystal Light, Pedialyte or Gatoraid (I know, artificial sweetener, but a few days won't hurt).
For the rash, as much air as possible. Oatmeal bath (Aveeno makes one or you can grind oatmeal to a powder in your blender). Desitin stings my little one when she has a rash. I switch to Butt Paste until the rash is gone. No wipes. Use plain water. Even the "sensitive" wipes might irritate him. I would rinse my wipes with water and that seemed to be less hassle than a washcloth. Or go right in the bathtub and rinse with water.
I feel your pain because I was in the same situation a month ago. No fun! Changing the diet worked immediately for me. Good luck.