My Iker is the same way, even after seven years. Things got much better when I changed the walking route, as it seems the first one I ever tried with him scared him. I also started to walk him out night, when there are fewer people around. I also avoid busy routes as best I can.
Some people recommend walking him in a circle when he stops, just to refocus him. You can try that.
I'm working with a behaviourist on Iker and this is what we've done. I walk him in quiet places using a 15' leash, which I gather and loosen depending on his needs. I let him decide where we're going, just to show him that he has a choice and to help get him to trust me. I encourage him a lot, but I don't force anything. I've been going to a quiet area of a local park. It's been very successful and he actually seems to enjoy it. He started to improve on street walks, but then I broke my foot and won't be able to walk him again until the cast comes off. Anyway, it's worked wonders for Iker.
Edited to add that I give him lots of encouragement and try to give him great treats, which he never takes because he's Iker