Beautiful horse, going kindly for a greenie.
Half halt, half halt, half halt! Get his hindquarters underneath his body and the level of his head will be raised accordingly. He looks like he is leaning on the bit, and you are allowing your upper body to be tilted forwards to compensate. You need to drive with your leg, don't allow him to rest on the pressure of the bit and your hands - he needs to support himself!
As Sarah Hershey mentioned, keep your shoulders back. When you feel his head dipping low, drive with that leg rather than closing your shoulders and tilting forwards. Also, although it is only one picture, it looks to me like he might benefit from a softer bit? Maybe a rubber bit or the like? Just a thought given the angle of his head...
All horses deserve, at least once in their lives, to be loved by a little girl.