Don't let him get his head down, and if he is doing this while leading him? Tap him under chin with toe of boot, at same time you tell him to quit, or get that head up.
Don't let him do this anywhere at all.
But the time to stop it, is before horse gets its head down, not after. Pay attention to what horse is thinking, and correct it by picking up on your reins, or telling horse, NO, and making it speed up some.
It shouldn't take but a couple of corrections on ground and from saddle to stop this.
Horses make me a better person.