How can I get my horse to walk while lungeing?

Well I just taught my horse how to lunge with a lunge line and he is quite good at it, I am able to get him to trot and lope, but I can't get him to walk. I tried using the command I use while riding him, I tried pulling on the rope a bit and saying "easy", and I tried putting the whip in front of him and saying "easy" but they don't seem to work. I was wondering if anyone would know any ideas on how to get him to walk? Thanks :D