What a terrible thing to happen! I sure hope someone can 'catch' him and contact that owner to get that boy home!!!!! Sad to hear of these things happening due to a failed clasp on a leash or collar. Just a reminder for us to not only care for our dogs, but to ensure the integrity and safety of the leashes etc.
A good "safety" exercise to practice is a command that tells your dog treats are in the offing, and use it at irregular intervals when you are walking. Combine the command with a total release of pressure on the leash, preferably when your dog has firm tension on said leash. Then if something unexpected happens you have a built in reaction of the dog to look to you for a reward. This won't work if your dog is pursuing a major distraction, but should give you an edge if you have an equipment failure in otherwise neutral conditions. (you can carry this one step further by "accidentally" dropping the leash in a controlled area, or with a light line attached for safety, and practice until the dog turns to you when it feels a total release)