NO no no, do NOT just put a muzzle on him and throw him in a dog park, if he is dog aggressive, he does NOT want to play with other dogs, he wants to HURT them. I strongly STRONGLY urge you to do tons and tons of research on shepherds and dog aggression before doing anything much else with him. Your previous GSD sounds like an amazing dog, but you need to remember to not compare him to your new one, he's a rescue with problems that you need to see if you are up for the challenge of, he is not and might never be like your old dog, shepherds are hard dogs, not all are going to be as amazing as your previous. Your new guy is going to need so much training, structure, time and energy to get him to a a good place to be the dog you want him to be. As for that trainer, he sounds like a dud, and if you did hire him he would do a horrible job, is there a local Schutzhund club near you? (that means protection work, like police dog type training) the trainers will be extremely experienced with german shepherds, and dogs like yours, and I'm sure can help you work with him, his aggression and obedience. you really REALLY need a trainer to help you, i agree that some trainers, like ones at petsmart training classes or something really don't know anything about training, and that with a little common sense and research the people who go to those trainers could easily train their dogs on their own, you need a REAL trainer, a dog behaviourist even, which is why I mentioned a Schutzhund club, the trainers there work on the most difficult high drive working dogs that other little joke trainers would have no idea what to do with, thats the type of trainer you need, thats the trainer that will help your dog, they can read the body language of a dog and know what they are going to do before the dogs do it. He sounds extremely smart and trainable, since he is already so good with the ear medication, he needs a strong trainer who is extremely experienced to guide him into the dog you want him to be, those trainers won't care about him being 90 pounds and having no history, they would work on him and know exactly what he needs. I have been training with a really good trainer and have been researching and working on crazy dogs like yours for quite a few years, being my dogs and other peoples dogs, if you have any questions i'm happy to answer and even help you find a GOOD trainer in your area