
Many ways to try and you will hopefully find one that will work.
The one i normally use is to walk backwards calling the dog to heel. I then praise and set off again. Then next time the dog pulls i walk backwards again.
Also there is just stoping dead, calling the dog back into heel and then praising and setting off again.
The next one is one i normally only use on VERY stubborn males out in a big open space. I simple give the dog one command 'heel' and i keep changing direction without warning. everytime the dog is in the correct position i praise.
Dont forget that whenever yoiur dog walks nicly on a slack lead just how you want then praise, stop to have a game, or anything that can be fun!
Have fun and i hope he becomes better soon
Rox