Can you post pics of it. If it is for sure a sperm plug then he will hopefully expel it when he poops. Mine usually just rub them off when they are walking around. If that doesn't work then you can try spraying water on his rear end and then gently pulling on it from side to side until it comes off. You should continue to spray water on it while you are trying the help it out. We had one that recently had a stuck sperm plug and when I tried to pull on it, it started to bring out his hemipene so I just left it alone and said I would give him another day and then take him to a vet if I had to. He ended up pooping it out by the next morning.