Your dog is maturing, so it may only be maturity and dog/dog aggression is just rearing its ugly head. But I doubt it. I suspect the new environment is creating/escalating it and I agree about invisible fencing. Your dog knows he is trapped, but yet other dogs can get it. Restrict his outside time to when you are with him, on a long line, fence off. A muzzle will help avoid bites, but won't change the behaviors.
Some dogs cannot be fixed when it comes to interdog aggression. Management, not curing, may be the only goal. But because this is new behavior, I would get a vet check, ESPECIALLY a full thyroid panel run since thyroid issues can cause aggression. Neutering helps some, but is far from a cure and sometimes if dogs are older helps minimally if at all.
Ask your vet about a veterinary school near you with a behaviorist, if possible.
PetMD: When Your Dog is Overly Aggressive Towards Other Dogs
Colorado State University: Effects of Gonadectomy on Health, Behavior and Performance of Pets
University of California Davis: The Truth about Aggression and Dominance in Dogs