Are you giving him a "stretch" phase after your flexions?? What should be happening is that you flex him (with the wrist only) and the other hand should be willing to give quite a lot. When he "gives", it will be a slight feeling of relaxation on the bit, they you need to feel like your reins are broomsticks and you are pushing the contact out, the neck longer and applying leg to activate the hind. If he is good with it, a slight tap on top of the croup with a dressage whip if the leg aid doesn't work should motivate him. Keep flexing both ways L&L (ie. Inside and outside flexions, both ways) until he is more supple. Focus on creating tempo changes, doing interesting lines and really stretching the back and activating the hind in the warm up, in all three gaits.
Once he is really stretching to the contact, your warm up is over. Give him a walk break and come into the work. Remember that even in the higher frame, the length of the spine should not change, you should still be giving after every rein aid (think of the broomstick reins) and that you should only have one active flexing rein at a time, while the other follows and allows the flexion. In between the flexions, keep your fingers active in the conversation and still giving if he chooses to take contact. The softer you are up front, the more willing he will be to move forward because he will feel less restricted.