In case of a planting, start flexing him and, when he moves willingly, encourage a step. Praise extensively, repeat, but don't overdo it - you won't be able to force him to move anyway, if you want to form and keep a willing attitude. There was a video posted here somewhere about this exact problem, it was really good, but I can't seem to find it anymore - maybe someone else will be of better help regarding this. :)
I have come a long way, to surrender my shadow to the shadow of my horse.