When you see other dogs, turning around is a good option, but making your dog sit while other dogs are nearby could be very stressful and difficult for him. Instead, when you turn around, try to get to a safe distance away from the other dog. There is SOME distance that your dog can be from other dogs without fixating on them, even if it is a football field or more. You want to try to keep him at this distance so that he is able to look at the other dog without fixating and will be able to take treats from you and pay some attention to you. If he does see the other dog and starts to fixate or otherwise react, simply move him to his safe distance and try clicking/treating from there. You will very gradually be able to decrease this distance.
Another option is using Behavior Adjustment Training, which teaches dogs to give polite signals that mean that they want to get away from the other dog, and they also learn that you will respect what they are telling you and you will give them the distance that they want. BAT works best when you have set up the situation, such as having a friend with a calm, friendly dog meet you in a big, quiet field, and you start out with your dog far enough away that you know he will not fixate. Once your dog notices the other dog, stop moving forward, and wait for your dog to turn his head away from the dog or sniff the ground, then tell him "yes!" or "good!" and jog about 15 feet away. If your boy gets fixated at any time, just go back to a further distance.