Do not let your dog off the lead wherever there may be other dogs until you have sorted this. Its not fair to other dogs or to him. Enrol for training classes to get him socialised and teach him to come when called. This is an accident (no, that should be incident) waiting to happen and if he ran up to one of mine and did it I would not be very pleased to say the least. As Lisa says, perhaps a behaviourist would be able to advise but you need to do some work yourself with him.