Although they know he is friendly, the other dogs and their owners do not know that, and their dogs could be aggressive, so it is not fair to them or the dog to allow this to happen. He should be kept on a lead (try a long flexi lead) until they have taught him to come back when called. They need to either take him to training classes or spend time training him with a long lead and titbits encouraging him to come a short distance at first and then gradually increasing it. When they are confident he knows he has to come when called, they should let him off in an enclosed area and call him back after a short while, rewarding him if he returns. Its no good waiting till he sees other dogs as his mind will be on them. He has no way of knowing what they want if they just let him off and then chase after him.