My 3 year old son keeps getting an itchy bumhole?

Can see his doctor. An itchy anus in a child can be due to irritation at the anus, or a rash there, which can be from having pinworms. The medicine for pinworms is vermox, which a doctor can prescribe. The doctor may also want to look at the anus to see if there may be another problem. Sometimes diarrhea, and other times hard poops, can cause anal irritation too.