One thing to be aware of, and avoid, is letting him drop the contact when he ducks. (Sometimes if you turn him immediately, you will keep the contact, and also remind him that his act was not what you wanted.) I would do a little yielding work first on the ground: hold the bit softly in his mouth, and guide his head to the left, then to the right. This is to initially get him used to the feel of the bit on his lips, not the bars.
Then do the yielding in the saddle. I've seen horses (like my own) learn high-headedness or behind-the-bitness because of being started in running mertingales: there's too much pressure on the bars of the mouth. It hurts!
Working forward helped my horse, and lateral work helped her learn to accept the bit. And I had to be patient, patient, patient.