IMHO, a "Dog Trainer's job" is not to teach the dog how to behave, but to teach the dog's owner how to get the dog to behave in the manner they want it to. Almost everything I learned (over the years) relates to the energy that the dog perceives from it's owner. In other words, if you are relaxed, the dog is as well. If you are tense, the dog senses that tension.
If your dog is misbehaving when another dog passes by on a walk, then stop before they cross paths, put your foot down on the leash (so your dog cannot do much more than sit or lay down). Give your dog a command, "stay", "wait", "easy" (whatever you prefer). Praise your dog. As the other dog approaches, repeat the command in a calm voice, alternating with praise. Since your dog is on a short leash and your weight holds the leash more securely than your hand and arm, your dog shouldn't be able to lunge at the approaching dog. If Boone is growling, repeat the command or just say "no". Perhaps, "no, easy"... "good dog". Your dog may be stressed the first couple of times. If you feel like you need to, crouch down and pet your dog. Do not hold your dog back! Just pet and praise. Let them know you are both "ok".
It will get easier. Some of the primary things to focus on is your own anxiety about the situation and your confidence that you can be in control of your dog. It's okay to tell the other dog owner that your dog is a little territorial, you are working on it, and that you are not comfortable introducing the dogs to eachother just yet. (i.e.; Please keep walking)
I feel like this is a good place to start. Training classes may be worthwhile, but remember, they are meant to train you how to interact with your dog, not to turn your dog into the perfect pet.