Hey buddy, don't worry. I raised 3 dogs of different breeds and all of them got some growth like that at some point in their life and were check by the vet. Some of them had them removed some didn't. None of them had health problems because of it and all made it to past 13 years old. As of right now we as humans have sis all over in the inside of our body. We all have a lot of them. If you ever get a cat scan you will know this. Just take him to the vet, but don't worry, your dog will most likely be fine.