1. Do t worry about his head and neck - it will come when he starts engaging his back and using his hocks
2. Have gentle steady contact through the reins
3. Sit tall, shoulders back, head up looking forward
4. Squeeze with your legs and think 'up' and get him moving out, maintain steady rein contact
5. Keep squeezing
6. Have him moving out ask, ask, ask,
7. As soon as you 'feel' that lift, his hocks will engage, his head will lower and he will feel like he is pushing you up in the saddle
8. Remember to sit deep but not hard in the saddle
9. - this is key - when you feel him lift, left him go for a few strides and gently release for a few strides and ask again, if you don't offer that release he will have no incentive to do it for you.
As time goes by he will be gaining the strength to maintain that arc in his back, engaged hocks and abdominal lift and he should feel like butter in your hands.
Hope that helped, it's easier said than done but is a wonderful feeling when you can get a horse there. One noticeable thing is you will see the wither 'lift'.