Hope this advise helps

Try and find a good training class or puppy socialisation class this way you can introduce him in a controlled enviroment.

If not try and find someone who has a dog that's bomb proof. I used to have a dog that other dog owners used to like using to introduce their puppy or nervous dog with because he used to ignore them and would play around them with out paying attention to the other dog. The puppy could then without being forced to interact go up to him in their own time.

Distraction training may also be another route. Have with you something that will make a loud noise eg Keys, rattle, pasta/rice in a bottle or training disc's as soon as he shows any reaction to the passing dog make the noise this will distract him and make him look at you. as soon as his attention is on you praise and reward

Let me know your feedback if you try any of this