Your dog sounds a lot like ours. Toby is now MUCH BETTER. He is able to play off leash with other dogs and goes to dog daycare once a week where he is completely relaxed. On leash he will still occasionally growl or snarl at another dog but is usually fine and the episodes are much less intense.
It sounds like you are doing the right thing but this is what we did. First, let me tell you that we have done A LOT of training with him. Our trainer felt that Toby was feeding off my anxiety and that he would be more relaxed if I acted relaxed and also was in charge. To be in charge, I hand fed him for about two months. This means instead of, here's your bowl of breakfast (and supper) now if your good you get a treat, it's work for your food. I would take his breakfast along on a walk and reward him for walking well on leash, sitting at the corner and being near another dog without freaking. This made a huge difference. We also did the desensitization training with the professional trainer (where he is introduced to other dogs at his comfort distance and rewarded for getting closer without freaking). Once he was doing better, he was ready for daycare and that also caused a huge improvement.
One thing I would suggest is to figure out his comfort distance. Take him to a dog park and see how far he can be from the other dogs while remaining calm. Start at this distance and just reward him for doing nothing at that distance. If you want, you can reward him for sitting or lying down or some other action. Clicker training works best here if you are familiar with it. Next, you need to move VERY GRADUALLY closer. Praise and reward. Don't move too quickly or you will set the whole thing back. It would be great if you could get him into a class where he could practise being near other dogs. You might even ask to take him there and just have him hang around at a safe distance and again, only gradually get closer. The key is not to hurry this and also not to use punishment or stern corrections. When we pass another dog I always talk happy talk to Toby and click and treat several times. This means he associates other dog with good things from me. It is important that you have a good recall with your dog and teach him good leash manners (hard with a puppy). If you have trouble controlling him on the leash, I would suggest the sense-ation harness (the one with the chest ring). It is more comfortable for the dog than a gentle leader or halti and gives you much more control on the leash.
I hope this helps. Let me know how it goes.