Hi, have you tried to use some kind of treat for your dog? Take something with you that he likes very much and show it to him. If he smells it he will proabably follow you. You can train that at home beforehand. Walk him at home the same way you would do outside. Call him and show him the treat (only a very small one will do!). If he comes he gets it. Repeat it again and again. And then go out and try again. Maybe it helps. If not, write again and I will think of another solution.
In the meantime
all the best!

Michael