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Great question! Socializing dogs with other dogs is a very important aspect of training. I recommend starting slowly, perhaps with a friend's dog that you know to be calm and easy going with other dogs. When introducing two dogs I've found that starting with a leash walk (both dogs on a leash), walking side by side until they "get over it" prior to any off leash activity. This can be challenging in that some dogs are more reactive/defensive on the leash than off, so you'll have to see how it goes with Jack as you get to know him better. I can't stress enough that the temperament of the dog you choose to start socializing him is key- it needs to be a dog that stays calm even when another dog is upset (hopefully you know at least one dog like this!). Short, positive interactions are encouraged.
As far as the fence fighting behavior, this can be very hard to change, particularly if the other dog does it too. All I can recommend there is using a leash and treating him when he remains calm near the fence. You can work the new sit/stay training in as well. Ultimately if you can convince him that he will get rewarded if he doesn't react or if you offer him something more appealing (a bone or toy, for example), he should grow tired of this behavior.
Oh my gosh if he isn't the most precious thing ever! You are so lucky to have found this little angel.
I'm not sure if you meant to open another question for this or not. If not, please contact customer service and they will take care of it for you. I wish you and Jack the best! Good luck in your training. :)