Thank you for your query,
Your child is having Gastroenteritis
and because of frequent loose stools he got nappy rash.
And your child requires medication for diarrhea like probiotic, oral rehydrating solutions and zinc supplementation.
You should give sips of oral rehydrating solution to replace the losses of water and electrolytes in stools and to prevent dehydration
For sore bottom, gently clean after stool passage, allow it to dry, change diapers frequently and you can apply zinc oxide
Consult your Pediatrician
for assessment and examination and for medication.
Hope i have answered your query.