He looks like he's tensing up in the neck and falling on his forehand, behind the leg as others mentioned. Push him up! :) Let him know that he doesn't have to suck back or get anxious about contact on his mouth. Really squeeze, squeeze, squeeze at the trot to get him moving out... that will help him relax.
As usual you look great! However-- see how you're sitting in that last photo? You're sitting deep on your seatbones (pelvis forward, and that lovely long leg!)? If you can sit more like that during your rides and learn to drive your horse forward with your seat, we will run out of ways to critique you from a still photo!