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Hi. My name is***** Congratulations on the new puppy. Is Sam eating and drinking well? What breed is he? Can you take a picture of his back side and post it for me to view?
That is good that he's eating and drinking well. His rear end may be swollen and red as he's had diarrhea that can cause irritation back there. The other possibility is that the other siblings were nursing / licking / chewing on that area. I've seen this before too. With any new addition to a house like this, the best thing you can do is to get him in the next day or so to your vet so they can check him over. They can make sure health wise everything is as good as it can be. They can check him for intestinal parasites, deworm him, check for ear mites, listen to his heart, and get his vaccines set up.
That perirectal region looks very angry. It looks like a moist dermatitis. He could have had diarrhea / fecal material on that area and it kept the skin moist and irritated. Cleaning that area up and getting it patted dry may help to quiet it down some.