I would not blame you for getting rid of this dog. If you don't feel equipped to deal with this situation alone, that does not make you a bad person. You fully expected a dog to be a lot of work, and you're taking dog ownership seriously, but you weren't prepared for serious temperament issues and don't have the time and money to properly manage this dog. Not everyone can handle that kind of thing, and that's okay.
The problem with rehoming him is that it's going to be very hard to find someone who wants to take on a large, young, problem dog. If the dog is truly aggressive, that's a huge liability (for both the new owner and potentially for you, if the dog were to attack someone else's dog). I would think about getting a second opinion from another behaviorist to make sure this dog is actually aggressive and not reactive (I'm not sure that leashing a dog to a wall and making a stuffed dog approach it is a good test; I suspect my AKK would be frightened/aggressive in that situation, too, feeling as if he had no means of escape, when otherwise he'd probably back away). If he is indeed "very aggressive," euthanasia may be kinder than rehoming. But I am not going to tell you what to do; I really don't know enough about this situation. I definitely recommend a second opinion, though.
The problem with rehoming him is that it's going to be very hard to find someone who wants to take on a large, young, problem dog. If the dog is truly aggressive, that's a huge liability (for both the new owner and potentially for you, if the dog were to attack someone else's dog). I would think about getting a second opinion from another behaviorist to make sure this dog is actually aggressive and not reactive (I'm not sure that leashing a dog to a wall and making a stuffed dog approach it is a good test; I suspect my AKK would be frightened/aggressive in that situation, too, feeling as if he had no means of escape, when otherwise he'd probably back away). If he is indeed "very aggressive," euthanasia may be kinder than rehoming. But I am not going to tell you what to do; I really don't know enough about this situation. I definitely recommend a second opinion, though.