A long back means he will jump flat and not jar the rider out of the saddle. However, if he has a steep shoulder and the angle at point of shoulder is not open, he may hang his knees over fences and that can be dangerous.
His back appears to not slope down steadily from the high point at the croup to its low point, but rather has a slight bulge upward just forward of the croup...you should be able to google roach back horse for a better explanation. His doesn't look real bad, just slight, and again it could be the picture...
Ah, I get it, and thanks PintoTess :)
Right now he is lacking in an real topline and he needs some groceries and muscles (I just bought him from a bad situation) could that make it seem like he has a roach back?