hi there,
Have you tried lungeing him in a shambon?
I have had really good results with that, as it helps them develop the right muscles due to correct head and neck position, without beeing harsh on them in any way as it works due to pressure on the poll.
I used it on a fresian cross TB that was brocken in wrong and had to learn all over again! Try lungeing him in one for 2 to 3 sessions a week, make sure that it is fitted correctly!
Also keep a good contact on the outside hand for him to lean on and spunge him softly on the inside when you ride. Young horses are like little kids that need to know that someone is there holding their hand as they cross the road.