I think you are doing a pretty good job with a green horse. Uma said it best with regards to your position. I actually think the lower leg sliding back and the ears being pinned are related.
Looks to me like he has taken off very deep into the jump, probably surprised you some which caused you to grip with your knees and hence lose your lower leg. If he is unhappy (not my horse, hard to say) then I don't think it means he hates what he is doing, this jump was just a little awkward for the both of you.
He obviously has potential but if he were my horse, I would be doing lots of gridwork over smaller jumps, teach him about distances and balance over fences. Add height in gradually when he is consistently jumping beautifully over 2'-3'.
All horses deserve, at least once in their lives, to be loved by a little girl.