Thank you for that information.
What I would suggest if possible is to make sure he is not in any pain which is furthering his aggression. This is a breed that is prone to hypothyroidism as well, and this can play a part in sudden aggression. I would consider a blood test be run for this. Generally this comes on after one year but closer to two years.
Depending on how you handled it when he was a year and a half old, would determine his path.
When you have a dog that wants to be alpha it is important to continue training on a daily basis in a positive manner as this reinforces your role in the home, this is not to teach a command he knows already. It puts you at the top so to speak. Dogs with this personality can't be given an inch, consistency is key through the dog's life.
I would also want to know if there is any possession aggression going on, especially with the other dog around. Take notice if he growls when you want to take a valued item from him or if he growls at the other dog if there is a valued item in the room. A valued item can be from anything to a bone, to space.
If this is happening then he also has some possession aggression.
Because he has been this way for a while it is going to take time to retrain him.
This can all be done in a positive manner though daily clicker training and the learn to earn program. The learn to earn is likely what your previous trainer was telling you to do, and with dogs that feel they are above all in the home, this needs to be done on a daily basis and thoroughly. Meaning it is more than just ignoring the dog when it wants attention. It means putting its daily dose of dinner high up and using it to train throughout the day, even if it is a few minutes of come, sit, stay then praise and reward. The more you can put the learn to earn in the training program the more he will begin to see you as a consistent leader and will stop trying to take over the home front.
As for walking him, he need commands in place such as a firm 'leave it ' so when he sees that squirrel or other dog, when you give the leave it command, he puts his attention right back onto you awaiting the next command. This is where your command watch comes in. Watch means the dog should be looking directly at your face when this command is given.
All of this has to be restarted in a non distracting area, so that he first sees you in charge. Then you take this training to the back yard, then to the driveway then to the street. The more the distractions, the better value the reward has to be. So outside in the driveway or the street you will need to switch to hot dog slivers or chicken slivers.
I will direct you to a few links for clicker training and the learn to earn. This has to be in place by the whole family daily.
I know this is a lot of information, but once you get going and make it daily , it will become routine to you.
Clicker training/positive method training/ print off
video’s to see how clicker training is done, scroll down to videos http://www.clickertrainusa.com/clicker-training-videos.htm
clicker maintaining focus http://www.clickertraining.com/files/auto-eye-contact.pdf
Dr sophia yin learn to earn https://drsophiayin.com/blog/entry/the-learn-to-earn-program/