Why does my dog always have a 'red rocket' and how can i get him to stop?

My dog had the same thing when he was younger also not neutered. I can honestly say I don't notice it anywhere near as often so I am sure it is happening way less. He is still not neutered and he has never been bred. I don't think anything can be done about it. It usually is more noticeable when the dog is sitting so you can tell your dog to stand or lay down.