I am by no means a good rider and I don't know how his hip would affect his overall position without being able to see his range of motion so excuse this if it's all rubbish!
I've taken a few stills, I hope that's ok?
From the video he seems to be carrying his position with his hands. Like he's leaning on them which causes them to be high. He's also looking down coming into the corner, pushing his position forward.
Again, leaning into the corner and looking down.
I'm not sure how much he'd be able to correct about his position given his hip problems. From looking at him he's sitting on his bum and not back on his seat bones which tips him slightly forward. His shoulder would ideally be more free and open with a bigger chest which would allow his hands to come down.
The saddle seems to be on the small side for him too, a bigger one would help with his position but I don't know many people that could afford one saddle for each rider!
He's doing incredibly well and should be very proud. I'm an absolute shambles on a horse after decades!