The back area is part of the topline. Lack of topline can happen due to age, lack of proper riding, lack of quality protein (amino acids), or a "hay" or "wormy" belly pulling down on the back.
I would deworm him thoroughly, including something for tape worms, like Zimectrin Gold (Ivermectin + Praziquantel). Add to his diet 1-2 scoops of alfalfa pellets (using a 3 qt feed scoop) or 1 or 2 flakes of Alfalfa hay. Alfalfa has high quality protein and amino acids, needed for muscle building.
If he's ridable, start riding him more often. Encourage a lowered head while driving him from behind, until you feel his back come up under the saddle. This will encourage him to use his back muscles more effectively. This is difficult though, so don't expect too much out of him for a while. Just keep asking and reward him everytime he gives a good solid "try."
When you do ride him, use a good thick saddle pad, perfeably one that's shock absorbing. He won't want to use his back muscles if the saddle feels uncomfortable. I like to use a Skito pad
for horses with a lack of topline, or a ThinLine pad
over a 1/2-3/4" felt pad.