Correcting the shoulders on a turn

This boy makes the first turn ok, but on the second turn, he does not listen to my inside rein asking him to keep his shoulders up....so I tap him on the inside hip with my whip. When we get to the other end of the arena, I tap too hard and he reacts...which I need to be careful of because I like him to stay relaxed. This boy has good balance. He is not as strong as he should be to carry himself and the overcheck is super loose. You will see it bouncing. I could help support him with the check, but I want him to be responsible and not to lean on the check.The video will take a bit to upload, but it will be at this link.