I think it's wonderful what you have done with him so far!
One of the most important things to remember is: Does my horse know how to give in to pressure?
It sounds like he does if you can give him a cue on the reins in order to drop his head for you. However, in his mind, he is giving you a response to your rein aid by dropping his head. You are asking him for something and he is giving you what you want by dropping his head and then lifting it once you remove the "pressure" cue.
My advice would be to help him out and keep a slight amount of pressure on your inside rein, just enough to maintain contact, but not overly strong as to bring his head to the inside (in extreme). In order to maintain the head set, we must give them enough hold on the rein to support their bodies if they are unable to do it themselves.
You can also practice this on the ground with him. With a lead line or even reins. Put downward pressure on the rein until he drops his head for you. The moment he does, release the pressure.
Or even mounted you can work on neck pressure. While he is walking, encourage him to drop his head by applying light pressure to his neck (just above his withers). The moment he releases down, start to scratch and love him. Continue this with him until you see consistency.
Hope this helped!