Show him the way to stretch into your hand.
When you first ask your horse to stretch - as illustrated - show him the way by taking your upper body forward but also taking your hand forward, after that it's just a question of low set, giving-forward, gently inviting hands while the legs continue to ask him to march on and seek the rein.
You'd be surprised how many people find this hard and tend to retract the hand. You can never give enough in the stretch. The ultimate is where the horse has elongated his neck into a long, gradual curve and continues to follow the 'ask' of our hand. It's all simple but subtle and it should be do-able for all horses.