That could be a variety of things. Its really impossible to tell with the poor picture quality. It could be a sperm plug but it could also be the beginning of a variety of prolapses. It is generally not a great idea to pull on anything coming out of the vent. If you do this, be VERY gentle. If it does not come out easily, leave it alone until the next bowel movement. Keep him well hydrated and you can try soaking him. If you can't pull it out gently and it remains after a BM, see a vet.