It's actually normal for an adult dog to "swell". That is why you need to keep him as quiet as possible. Basically, it's blood pooling in the scrotum and the body will reabsorb it. However, you probably should call your vet. I had a foster that swelled like that and he had an infection. You can ice it as well. I sat for 10 minutes at a time, once every hour, with an ice pack on it for him.