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I hope you have luck with the trainers. In the mean time, I noticed you said your SO is afraid to take him out because of his size. I have a reactive 80+ lb German Shepherd and I'm about a 115 lbs. I understand. A couple of things:

Do you have a secure, front clip harness you can use to control your dog? If not, I would suggest investing in one. It feels a lot more secure vs just using a collar that your dog can slip out of.

I recently started using two leashes to walk my GSD. I have a short bungie leash, like this, which I keep clipped to my hips. You can use a heavy duty carabiner attached to a belt or fanny pack.

Then, I have a second flat leash which has an extra handle at the base of the leash, like this, so I can get a really good grip on my GSD and pull him away if I absolutely have to. I keep the flat leash looped around my wrist, then looped around my thumb like this, close my hand into a fist around the leash and hold it tightly to my stomach.

This has increased my confidence ten fold because I know my dog cannot pull away from me if he's attached my hips, and I can still easily control him using the second handle on the flat leash. The second handle on the flash leash is a blessing for tight areas like hallways or stair wells in the apartment complex (Avoid tugging on his leash if its not necessary, this can make them more anxious).

Other tips:
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Make sure you are playing with your dog inside the apartment as much as possible to reduce his stress, anxiety or tire him out. (Or take him the park and try to really tucker him out) Try to walk him at odd hours (like 5am, 12am) to reduce the chances you will run into things that scare him. This sucks, but it is very important that you avoid stressing him out as much as possible.

Practice your emergency exits. If you are in the hallway and see something scary, you can get away without your dog reacting and stressing out. Start in the apartment with his leash on, and practice saying "Lets go!" in a playful voice, do a u-turn, and playfully run the opposite direction. You might need to use your hands to get your dogs attention or use treats to lure him into a u-turn. You don't want to be tugging on your dog. You want your dog to think this is a fun and exciting game that is way better than facing and barking at whatever scary thing is at the end of the hall or sidewalk.

Instead of just feeding your dog cheese when he sees something scary, try turning it into a game of "Find it!". Again, start by practicing in your apartment, throw a treat a couple feet in front of you and tell your dog to "Find it!". You can start using this outside, when you don't see any distractions. Then, if your dog is far enough away to see a distraction but NOT REACT, you can try playing "Find it!" to distract them and reduce their stress. You can also use "Find it!" to get them to turn around, or throw treats at your feet so you can more easily get their attention.

Right now the goal is to avoid triggers as much as possible so if in doubt, do an emergency exit and try to get your dog out of the situation by doing a playful "Let's go!". You can play "Find it!" when you are far enough away. If your dog notices a trigger but is far enough away that he isn't reacting yet, try playing "Find it!" to distract your dog from the trigger. Always try to set him up for success, don't do it if you know he's gonna do bonkers.

Also, experiment with high value treats or rewards. Does he have a chew bone he really loves? Try bringing on the walk to distract him. Try really smelly, wet treats like hot dogs, liver or chicken.

Hopefully this will help until you maintain the situation until you can get some hands on training with a behaviorist.