Roach back is he has an elevated curve to his topline (more than the average). It could be that his croup "top of the butt" is too rounded, giving the impression of a roach back. Either way it will be faulted in a show ring. His loin (top inner point of hip) moves "off" also. This whole area could be from one structural flaw, I just can't put my finger on which one with the video shown. When moving in the ring, you want a big reach forward "flashy mover". He doesn't have that. I think it is in part from his hip issue but could also be from his pasterns (ankle). The rotate down toward the ground instead of straight up from the rear of the paw, which causes his paws to flare out "splay foot" in compensation. This can be caused by kennel life (small pen on rocks/concrete), poor nutrition, or structural flaw. If it is environment, you can correct the issue, if it is structural you can't.
I am not an expert on DDB though. Maybe it is a "wonky" growth stage, and his rear end will catch up. I am not familiar with their age/growth pattern. I know DDB are more rounded in the croup than other mastiffs, so it may be a possibility. The breeder should be able to tell you, as they will be familiar with their lines. I personally would not choose him for show quality, I would pet him out.
