Just be careful that your not asking too much. I had this problem with my horse for a while, but everything comes down to the rider.
remember the key thing. Pressure, release, reward. The release is his reward. Which means, ask him to bring his head down, push him into the bridle, and once he's there hold your hands quiet and stteady and let him do his own thing! Make sure your not comstatly 'nagging him', or he won't understand what you want him to do.
Make sure he keeps moving forwards. Lots of bending and flexing. Flex his neck to the inside of the circle, then soften him and slowly bring his head back straight. and repeat, and repeat.
hope this helps a bit