My girl Bailey statures a lot...she did it constantly when we first got her and another grey owner came over once a week so we could go for a walk with another pup.
Thank goodness the heaviest she has reached is about 64.5 pounds because I had to carry her a lot. Here's was strictly fear based. Now she does it and I just have to coax her and tug her leash quickly to get her going. When she does walk well, I give her a treat but never while she's statuing.
How long have you had him. Bailey was OK for the first 3 days because she was shell-shocked but once the shock wore off, this became a huge problem.
Try not to fuss too much so he doesn't see it as reinforcement of his behviour but make sure you are the leader. For example, if you decide to turn around bring him around in a circle so he thinks he following you even though you end up going in the direction he probably wants to go.
I really had to learn how to give gentle tugs with purpose and sometimes two or three in a row but after 3 years she's a lot better. Perhaps you can walk with a friend who has a dog, even if it is a different breed. This helped us.