Well, this is not an easy problem to correct, as the horse has learned to escape the action of the bit by going "behind it".
We often see this kind of problem in horses that have been started with draw reins too early (used incorrectly or totally inappropriately)...or have been aked to "come onto the bit" INCORRECTLY (hand back) - OR, and this is more common: A rider who has not yet developed sensitive and feeling hands that offer a contact rather than "taking" one.
First of all, it is important not to try to raise then hands in the attempt to lift up the nose.... that doesn't work!
What you need to do is to go back to "scratch" and ride with a long rein a lot - barely a contact, so the horse can only feel the slightest elastic connection: One that can be taken forward - without any resistance coming from your hand. You have to provide him with the same "feel" consistently, so he can learn to trust it. Then you can ask him to step forward and into it without ever bringing your hand back.
It needs a lot of feel!
When I teach I always say something like "this connection should feel like you pushing a freely rolling shopping trolley (pram...)and you just connect in front. You drive from your leg when needed. Be not too eager with your seat yet, as a young horse easily gets tense if there is too much action coming from your seat. Build him up slowly.
Do a lot of "interesting" exercises: After you have established good steady rhythm you can slowly go for your "long and low" (he'll pick it up quickly, so be ready for him if he is ready to stretch down, so he does not "bump" into your hand. Then do different things - not just arena work. Include Cavaletti work and lots of relaxing outings... ride straight lines, forward into an always "receiving"...never "taking" hand...
Another thing to try is a "bitless" bridle! They are becoming more and more popular! If there is no bit... he can't go behind it!
.... and as we all know: "Control" has nothing to do with the bit.....!
There is a lot of info on them. - Have a look here:
[
http://www.bitlessbridle.com/]
Anyway. Good luck and let us know how you go!
Cheers,
Carola