From a standstill, shorten one rein a bit, life that hand slightly to apply light pressure to your horse's mouth, then just wait. You'll feel tempted to increase the pressure to get a response--but don't. Apply just enough pressure that your horse begins to look for a way out of it.
Then, the instant he drops his head even slightly, release the rein entirely and praise him lavishly. Don't worry if he moves about a little while you're working on this; just focus on the head dropping and release whenever he complies. As before, the first few inches will be challenging, so be patient. Once your horse figures it out, he'll begin to respond quickly and willingly.
Then practice the drop-head cue frequently on an ongoing basis, especially any time your horse becomes tense or fearful.