He shouldn't be positioned behind you when you give the command to walk. His head should be slightly in front of you and you should be inline with his sholder or slightly behind you.
It will take you awhile to adjust and it will take him awhile to adjust as well. DON'T get frustrated. Just as an artist continually works at their craft you too must continually work at it to succeed!
When you say walk, take a step with your right foot he too should start his walk. If he doesn't at those first commands then give him direction by reaching behind you and tapping him. Don't keep walking if he doesn't complie with the very first command.
Here comes the hard part....
If he is walking in your space turn around and wack him one in the chest with the crop or the lead line and say NO! and then after he settels down position your self infront of his shoulder and his head slightly in front of you then do the walk command; if he steps in your space again do the wack and say NO! and do the walk command again. You may have to do this several times. AND I just want you to know that for a time he may be affraid of you, but he will get over that once he realizes that this is how it will be and he will conform and respect you. Don't change the way you work him on this that will only confuse him and then he won't know what to do. Nothing worse than a confused horse
Keep on him and you will see that he will change with time and be more willing once he understands what you want. You have to spell that out to him since he doesn't talk our lanquage and we don't talk their lanquage.
Once you establish the ground rules he will look to you for direction on the ground and in the saddle, but you have to explain to him in REPEATED actions what is acceptable and what isnt.
Don't give up he needs you to show/teach him the rules and once you pass all the mile stones you will have a great feeling of success and a more safe horse to interact with.
EDIT: Once you've completed your walk with him and you put him away reward him NOT with food. Tell him he's a good boy and give him a couple of pats. Don't lather him with petting, but make it short and most of all SWEET. Remember to reward him when he does good. If your schooling him Don't stop what your doing to give him a pat, but tell him by singing "GOOOOOD BOOOY"! and continue on with the exercise.