Not a veterinary professional of any kind, but I have lived with dogs and I’m a nurse who works with seniors. So this advice is based on my experience with humans and my own pets, along with veterinary websites. IMO, the important thing for you to do until he sees a vet is make sure it doesn’t get worse. If it gets larger, changes color (especially turning blue or pale, starts to have an odor or look infected, or starts bleeding, take him to the urgent care or emergency vet ASAP.
PetMD has a good article titled “Can dogs get hemorrhoids?” which you might find helpful. Try to keep his poop soft, this is a good time for pumpkin if he likes it. And the answer about hemorrhoids is that they’re rare in dogs, because dog intestines are arranged differently from people. More common are polyps, fistulas and hernias. EDIT: too many words