I agree that male-female pairings are best, but I also think that same sex pairings can work just fine...depends more on the disposition of the dog than the sex. I would be concerned about Jax being aggressive to another dog if he has not been well-socialized with other dogs. You mentioned that he isn't familiar with other dogs, and I would recommend that you take him over to the breeders place or to another house with dogs and work on socializing him. He may not be aggressive, but he will likely be scared of other dogs, causing him to snap and growl, which typically leads to aggression. Definitely begin socializing him heavily with a variety of other dogs...even if he accepts the pup that you bring into the house, he may be aggressive to other dogs. (I'm a big proponent of heavy socialization at a young age and for the rest of a dog's life, as you can probably tell...) Good luck!
