Thank you for your query on Healthcare Magic.
Your child seems having gastroenteritis
The mainstay of treatment is to maintain rehydration fluids orally like Pedialyte
Monitor his hydration status.
Hives are due to some allergic skin reaction.
You can apply a calamine containing lotion externally over skin for soothing effect.
If there is itching give Benadryl
syrup in prescribed dose.
Consult your Pediatrician
if the symptoms are not getting better.
Hope I have answered your query.