What Causes Itchy Red Ring Around Anus?

Hi, My son has a red ring around his anus, and just had a bowel movement that I wasn't able to get to before he flushed it. He said he had blood on the toilet paper when he wiped himself. He is going to be 4 in June. He also has been itching his butt alot lately. Is this some sort of worm do you think?