Thank you for your query.
The anal region will not be red or have pimples
like bumps in the area due to holding of stools. There must be some other reason for it. It is advisable that you show your son to a doctor who shall examine him and guide you.
Due to passage of hard stools, a child can develop anal fissure
which is very painful while passing stools. By giving laxatives and/or using suppositories, you can make the stools softer and thereafter the process of defaecation less painful.
I hope I have answered your query to your satisfaction.