Desperate for advice!!

Help!! My son is on a antibiotic for a double ear infection. He has diarrhea so badly it’s pretty much just liquid. He seems pretty dehydrated but no thirst or appetite. We have kind of been making his eat and drink his formula. The rash came so suddenly and out of no where. He screams every time we wipe him but we are wiping him because of the diarrhea. We tried the most gentle wipes and even a wash cloth. We are currently using desitin max strength for him. It’s helping but there’s a few spots where it’s so bad his poor butt and sac are so raw and even starting to bleed.