Did you try the drunken sailor walk? The key for my horse was taking him a few steps past his comfort area each day. The problem was getting him to take those steps. We did it with drunken sailor walk.
When you get to that place where he won't go anymore forward, take both your legs off, or rather leave them hanging gently at his sides. Then take one rein at a time and get him to follow his nose. He'll take a few steps to one side, then drop that rein, pick up the other, and he'll wander in that way. The resulting S shaped walk will result in forward motion. Somehow the alternating reins seems to get their mind more focused on the rider too which is always nice. Try it. Nothing to lose!