If he stops, definitely don't swat his neck to try to get him to go; you need to engage his hip, and you can't do it from the shoulder!
When he does stop, you can take that time to practice yielding his hindquarters, backing, yielding forequarters, etc. Keeping his feet moving will help him realize that he can't decide when to stop; you are in control of his feet.
Make sure you carry a dressage whip, or handystick, or other tool you can use to tap his hind end and shoulder to get him to yield to you. When ever he gives to the pressure you are putting on, you need to release the pressure, as well. Practice yielding the hind quarters a few times (both directions), and then walk off like you normally would, and if he doesn't move with you, right away ask him to yield some part of his body, whether it's his hind end, front end, or all of it (by backing).
You can also use longing (if he knows how) to get him to unlock his feet; once he stops, immediately start longing him; you'll have to make sure you have a long enough lead, or use a longe line when you walk him, so you don't damage his legs. I always work a horse with leg wraps, especially if it's a young horse, so you might want to consider it atleast, if you don't already.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."