You need to 'get in first' with a warning, try making him sit or lie down if you see another dog aproaching, and say in a sharp voice "if you dare" and if he stays sitting and doesn't do anything then tell him in a calm voice "good boy" and walk on. You need to be very quick and ready for any other dog getting near, but by letting him you know what he is planning, he may think twice. It could also be that he thinks he is 'protecting' you and you need to make him aware that it is not neccesary.