Hi I've had the same problem with my cockapoo which I got at 5months old who had no collar and lead experience.
I've had him for two weeks now and has greatly improved by carrying him if he was scared when out but just let him experience the sights and sounds.
I would put him on the ground and he would just sit and not move so would throw lots of tasty treats in front of him and he would slowly move forward.
I found him walking alongside another well behaved dog help a great deal .
It was a slow process i let him take his time and gave lots of praise and throwing treats in front of him and rewards when walking.
If I wasn't having a good day with him I would just sit on a park bench and watch the world go by.
Two weeks later I've got a dog who enjoys going for a walk.
Good luck