Thanks for your query.
As mentioned by you you child is having diarrhea
for 3 days and now frequency has increased.The most important thing is to prevent dehydration
so i will advise you to take him to pediatrician
for evaluation,in the meantime give him plenty of fluids,give oral rehydration solution sip by sip after each loose stool
.give coconut water,banana,boiled potato.rice with curd,if he accepts oral fluid and tolerate them then there is not much to worry as diarrhea will take few days to settle,but in case oral acceptance is not good then he has to be hospitalized.
regarding redness in perianal area this is due to constant irritation of that area,and will subside once loose motions get better.
You son will be fine,