Seems like a risky purchase, but I do feel sorry for the horse.
What does the owner say is wrong with him?
I definitely see what you are talking about. It is very evident in all the ridden videos. The free lunging video I'm not sure. But even when he is not knuckling over (or whatever you would call it) he looks stiff in his hind end. I don't know if that would be hocks, stifles, or what.
I also wonder about saddle fit with a heavier rider. I am a heavy rider myself and I consider saddle fit critical. If he has back pain that could cause him to have trouble with his hind end. I had an Arabian whose back end would dip out from under him occasionally while being ridden and I was a novice at the time and didn't know why. Looking back on it I'm sure it was a saddle fit issue.
Have you tried running your fingers along his spine to see if he shows back pain?
The best thing to do, if you were serious about the horse, is get a vet's opinion. As a purchase I think he would be risky. Because it could be something potentially serious like joint problems. But it could also be something curable like a poorly fitting saddle. A vet would be better able to make that call.
I really feel bad for the horse, that he was pushed to canter when he obviously has problems. And he's also wearing a tie-down. He might be carrying his head high because he's in pain not because he just wants to carry his head high, know what I mean?