Irisheyes12- thank you for the medical term, I didn't get back here to post it right away... lordosis...
As for helping a horse's topline if they just have weakening muscles or a poorly muscled back, work with them on hills, backing, trotting over poles, get them working on framing up correctly and using their hindquarters... do you ride with your horse on the bit/flexing vertically? That is important for a good topline.
Poles and hills don't really help with the back part of topline... they help with the hindquarters.
The only way a horse builds up real nice topline is by working in a correct frame. By working in a correct frame, that doesn't mean head on the vertical. It means correct head carriage, /forward/ motion, and NOT your classic 'high butt, high forehand, sway back' that's in a lot of dressage now. For a horse to have nice topline, he has to carry you with his back and ab muscles... and he can only START to do that by riding him forward. No fast, just energy in each gait (and the gaits within a gait).
Even western horses need to be moved out--they look angular and out of shape otherwise... a little like your horse. He doesn't look terribly out of shape, just not being ridden to produce correct topline.