I have the same issue wth my guy. Transitions are key. Worry less about his headset and more about asking him to step up under himself. For my guy it helped to sit up like I was going to really pick him up but leave my hands steady. Keeping his mind on and "collecting" without actually touching the face.
Hills were also amazing. They help to force them to use their hind end without you needing to do more than needed. This making the discovery of their butt less like work and more like they found it on their own and discover it feels better to use it.
And ultimately make sure your tack isn't causing your horse to not want to use their back.
What do you mean by 'getting under himself' does he not meet his forehand with his hind feet in the trot diagonal? Is his back arched and neck raised?
Other than using poles and cavallettis great things are riding uphill since they push with their HQ. Transitions are excellent when done very frequently. Especially downward-upward transitions like walk-canter-walk. Exercises like canter-backup are very effective.
Also exercises where you extend and collect his stride.
If he has a really long back you should also do stretches with him to help his back muscles. (Like holding his poll unit he lowers his head straight to the ground, using a treat to have him reach sideways around to his barrel. There's a lot online about it.
Doing lots of bounces would help. It's kinda like squats for horses.