Drunken sailor walk! You definitely need to get "backwards" out of that horse's mind. As soon as you feel him start to get sticky, take your legs off, (not off his sides, just no pressure) and begin to alternate reins to get him to literally follow his nose to the left, then the right, etc. When using one rein, make sure there is no pressure on the other one. The idea is not to command the horse to go forward, but rather trick him into it. The mere act of following his nose gets him moving forward albeit in a zig-zag fashion. As soon as he "unsticks" go back to a regular forward walk and praise. Something about the left/right motion also seems to engage their brain inward instead of looking for all the monsters out there. Love the drunken sailor walk!!