So, I had a reactive dog in the past and one-on-one training was super important. She’d also bring her dog as ‘props’ to get our dog acclimated, as hers were non-reactive and very well trained. I have a few priceless things we learned below, sorry it’s long!
First, you should get the book ‘On Talking Terms with Dogs’ by Turid Rugaas. It’s short and packs a powerful punch because everything she talks about is also in picture form to show you the different nuances of canine communication which should help you spot your dog’s individual limits and allow you to tailor your training to his needs more effectively. Second, consider one-on-one training for dog-reactivity with a positive reinforcement trainer with lots of experience. It may be expensive, but the trainer is invaluable in pointing out not only what your dog is doing, but what you are doing that is contributing to the situation.
You’re basically putting your dog in a 0-60 situation and he doesn’t currently have the skills to cope effectively with the changing environmental stimulus, so you need to dial it back. He’s trained indoors, with zero distractions and lots of motivation (both on his and your part). The next step is taking him to the backyard and getting him sound there with focus and responsiveness. I like the ‘Find Me’ game and the ‘Focus’ game.
Basically, for the Focus Game you have your pup on a short leash and reward when your dog makes strong, focused eye contact with you. This is contrary to their nature (staring at a dominant pack member), so at first you need to reward fleeting, but focused, eye contact and build up their confidence in holding eye contact. If your dog really lacks confidence, start inside and off leash sitting on the couch or somewhere they’re most comfortable and relaxed. My dog will make eye contact for a long period of time sometimes, but she’ll be sniffing the air and you can tell her mind is elsewhere and she’s just feigning focus on me- DON’T reward that type of behavior. Both body and mind should be on the trainer (yours should be on your dog at all times, don’t allow yourself to get distracted). You can do this in 2-3 min sessions a day and he’ll get the point that part of his job is to remain focused on you and he’ll get the necessary practice to re-focus on you when distractions arise.
Then, after your pup understands Focus, you can work on the ‘Find Me’ game. Make sure your dog can only move 2 or so feet from your feet, so they can’t keep themselves entertained for too long. With your dog on a leash and in a slightly more distracting location, you give the command for your pup to sit at heal. Your dog should respond and promptly make eye contact with you for further guidance. If your dog does not respond and is getting its kicks out of sniffing the ground and air and looking around- do not respond and do not try to get your dog’s attention in any way (shuffling, coughing, repeating the command, etc.). You want your dog to voluntarily buy into listening to your command authority and following your lead, you don’t want to have to force them through authoritarian direction or repeating the command (teaches them to ignore the first iteration until you’re ‘serious’ or they feel like it.
The idea is that at some point, your dog will want to keep moving and exploring. The only way to get that positive input (movement and new environment) is to pay attention to its trainer. Once your dog refocuses, reward and move a bit. Then, when your dog is getting a little distracted again (but before they get overwhelmed), stop moving and give another heal-sit command. The trick is to never put a time limit on this exercise and don’t undertake it if you only have 5-10 minutes and you’re going to rush through. You have to have unending patience and attention on your dog so you don’t miss rewarding their focus and attention on you. The world is an exciting place! You can’t expect to compete with that if you miss the proper behaviors your dog offers. If your dog gets way overexcited and can’t reign it in, you change the distraction level by moving the training location, not by changing the rules of the game. Once your dog is succeeding at refocusing himself and rejecting distractions quickly and confidently, then you move on to a more distracting location and get your dog succeeding there.
Finally, with regard to dog-reactivity (notice that there’s a ton of precursor work you need to do before you even get to working with another dog in the picture). You’re not showing leadership to your dog and setting appropriate boundaries. The exercises that I discussed above are rebuilding his confidence in you to manage the external world and keep him safe/secure in it, without those foundations, he’ll be very unlikely to handle dog interaction situations in any other way than he currently is. There are a lot of steps between your dog seeing another dog and getting all the way to greetings! You need to address his behavior in between those steps so that he learns the boundaries. That book recommendation will walk you through what ‘calming signals’ polite, well-adjusted, confident dogs give off to other dogs when they are communicating. The trick is to teach your dog to read these calming signals and respond in kind. If you get too close in proximity, your dog will start exhibiting stress signals, which means you need to back up and give your dog the space it (or the other dog) is requesting. Instead, you’re pushing your dog into a social situation in which it lacks the confidence to handle calmly (and you’re probably feeding the cycle with your body language and behaviors too, which the trainer should help you to identify). Over time and with deliberate work on reading your dog and other dogs’ behavior, and knowing how your behavior is helping or hurting the success of the interaction, your dog’s ‘bubble’ should shrink to allow him to get closer and closer while acting and reading the situation appropriately. It takes a LOT of patience and an even-keel on your part as well. Good luck, hope this helps!