Due to his long back he will be a bit difficult to collect and get working off his forehand (see cantering photo.. yes, as you self critiqued, your position is forward and hands up, but he is all the way on his forehand too).
You have a lovely place to ride.. work him on circles and transitions to get him thinking about his hind quarters and trot him up those hills (let him blow at the top!). Work on balance.. and going from extension to collection and back to extensions at the trot using the half halt. Work on spiral circles.. starting large and working smaller and then spiral back out again. Pick a spot on the ground and trot him in nice round circles to get him supple and bending.
Horse has potential. Sit up straight and use your butt and legs to drive him forward and up into your hands.