This behaviour of your dog, of charging at other dogs is quite common for someone his age. There is curiosity, and there is also the need to show aggression, being the top dog, basically. However, like you said, while it is harmless in most cases, owners of other dogs take offence simply because they are protective about their pets as you would be about yours. There are a couple of things you can do to correct this. One is to take him to a trainer who has the required experience to be able to help you. Putting him through his paces afresh would help him overcome this habit.

Another option would be to not walk him off-leash in public spaces where other dogs are around. That is a preventive option you can adopt for the time being; however, the best option is to take him to a trainer who can help him overcome this habit. Your dog is still young and possesses raw energy, and the best option is to get him to harness that energy into other channels. A trainer will be able to accomplish this – overcome his habit of charging at other dogs.