It looks like you are "half halt"ing a lot with your hands. Your line of communication has to be very soft and elastic, and right now he is trying to get some elasticity by opening his jaw and coming behind the vertical. You need to become softer and much less reliant on your hands. Trust that he will stay round and on the bit in self carriage. This will also allow his neck to build some much needed muscle. Right now his neck is kinking and the base of the neck is pushing down, adding to the downhill appearance.
You need to sit into the saddle to half halt and really use your abs and back muscles to balance him and bring him to the pace or gait you want. The reins are only for a tiny bit of flexion and otherwise are decoration. Your elbows and shoulders need to relax and sink down, while your hands stay in front of the withers. Really drive your elbows to your hips in order to stabilize your upper body and keep your shoulders down and relaxed.