Sounds like his actual whoa cue is all right. I assume you're asking by giving the verbal command, sitting deeply, and pulling on the reins as necessary. As soon as he sits to stop, release the rein pressure. That's his reward. Good. Now he's stopped, with his head down and the reins slack. He starts to walk off. Nope. Pull back on the reins and push him right back to where he was initially, then drop the slack again. He moves off again. Not going to happen. Pick up your reins and back him straight back to base again, then once again release. Repeat as many times as necessary, without losing your patience or temper. He'll grasp the concept eventually.