When you are wanting your horse to "work on the bit", you must never try and pull and tug on his mouth to get him on the bit. A horse's head doesn't HAVE to be on the vertical for it to be considered "on the bit". It just means the horse is working actively from behind and is accepting rein contact, keeping a soft relaxed mouth.
If you pull his mouth, he will get confused because you will be kicking with your legs, telling him to go, but then also pulling on the reins telling him to stop. Do what Kayty said in her post, transitions, changes of rein, etc, to get him working actively from his hindquarters. Over time he will eventually start accepting the contact on the reins and soften and go into a nice frame, as long as you have gentle hands that don't tug him or move around alot, keeping them still and relaxed and able to move with the horse's head.
If you do it right, the horse will willingly take the contact and work nicely on the bit. Good luck.