When they stop take the opportunity to work on disengaging the hind quarters, disengaging his front end, backing them, or turning them on their haunches, you can also work on bending each side of their neck, by bringing the lead up to his wither and waiting for him to 'give'. The point is just to get their feet moving, without pulling on their halters, or whipping them foward. While you are doing this, they will get refocused on you, and will also decide that walking foward with you is easier than having to do all that other work. If he still won't move foward, just keep doing the 'refocus' work, and trust me, he will walk foward sooner or later, and you won't have to whip him into it; the advantage will be that he will be quite supple by the time you even start thinking about bridle or saddle work.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."