It sounds like your dog is not that confident around other dogs, you mentioned him being attacked as a puppy. Tell us more about his behaviour, is he fearful of these dogs?
It sounds like you need to get familiar with counter-conditioning, your dog needs it. Its not that hard, look it up and start using it. It is important to remember to take baby steps when counter-conditioning, its faster when you do.
Also have you tried walking your dog off leash? dog on dog encounters are usually drastically improved with both dogs are off the leash. It relieves much of the tension the leash causes, I would definetly reccomend you try walking your dog on leash, or a 25 foot lead, or at the least, give your dogs TONS of slack when another dog approachs your dog, I would not reccomend the gentle leader for this, it seems as though, once a dog approachs, you tense up, and try to "control" your dog, which is causing much of your issue.
If this doesnt work, dont be afraid to carry an air horn on you, an air horn is an easy way to deter another dog from approaching yours, or to break up a dog fight once it starts. The airhorn may give you the confidence you need to walk around without fearing dogs approaching, because youll know if they get into a fight, you can break it up quickly, just point and shoot.
Good luck counter-conditiong, stay relaxed when other dogs approach, give your dog al the space it wants, and use an air horn if your afraid/need to build confidence.