This is one of those situations where it is really hard to help unless you see the dog (aren't they all!!).
The first thing to do is to really think about yourself when this happens. Do you see someone coming and anticipate your dogs reaction? (I know I would!) Do you start to gather in leash and prepare yourself? Do you shout or apologise quickly to the "victim"?

In some way the dog is not interpreting your actions as a signal that the behaviour is unwanted and may be seeing them as actual encouragement and validation of what he is doing.

Try slowing things right down - set up some people you know to "meet" you out walking. As soon as the dog begins to react just stop, with a loose lead, ignoring both dog and passerby. Do not walk on until the dog is quiet. Do this ALL the time, simply stay as relaxed as you can and keep the lead loose - do not look or speak to the dog. The first time the dog gets closer to the oncoming person than is usual give him some calm verbal praise and a food treat. You must be sure that he is definitly closer - if he then starts to bark - ignore him again.

The slower and calmer you can be the better.
Good luck, remember he will take a while to realise that things have changed and you will have to persevere. Let us know how you try and solve this problem and what how you get on, please.