He does look to be a little downhill and it will make getting him to stretch over his topline a little difficult. That being said, you'll never build up the proper muscles if you work him hollow like he's going in those pictures. You look to be a good rider, so I'm sure your ready to turn things up a notch. If you fix your hand position, so that you have a straight line from your elbow to bit, it will go a long way toward getting your horse to reach into the contact and become soft in the bit rather than bracing against it like he's doing now. In those pictures, your hands are too low to create that line. Lift them up, usually a pinky's length above the withers, bring your hands forward so they are in front of the pommel, take up the slack, and that should create a more accurate line to his mouth. The "contact" itself should be just enough so that you can feel his mouth without pulling on it. (if that makes sense) Ideally you want your horse to move with his head slightly in front of the vertical. He looks like he has the impulsion necessary to drive him up properly from behind into the contact, so it's just a matter of shutting the front door a bit so he isn't running through loose contact. Start at the walk and establish the sweet spot of contact. Move up to the trot. Every time he starts to brace against the bit, come back to a walk for just one or two strides and then try trotting again. Any more and you're just letting him off the hook. Most horses get in after about 7 transitions and soften to the contact. The split second he does what you ask, soften you contact and reward him, so he knows that's what you want. He'll be using an entirely new set of muscles to do this, so be patient with him and reward him everytime he makes an effort. Good luck.