Thanks for your query
sore bottom is a part of previous infection with scarlet fever
I advise you to apply caladryl
lotion 3 to 4 times a day,pat area gently,do not rub it,give him plenty of fluids,fruit,salad,green leafy vegetables so that he will not be constipated.
In case symptoms persist get a pediatric consultation.
Wishing your child good health