I think that's probably a good idea ! My joePony was doign the same - scuffing his back feet square, unenthusiastic about going downhill. I was busy damaging his back with a poorly fitting saddle - it was too curved from front to back - it was just balancing there on his flat back. Having sorted him out a new saddle, and having the chiropractor in, I've carried on riding him, and kept him at a walk, but really working him at at the walk. He's tired out after 25 minutes, and so am I. Really good schooling at a walk will do wonders for you - well, I mean, when he's ready to take a rider again. I think you'll do great to start with if you do really good lunge work to build his back muscles up correctly.
Another thing you can do is the 'western' version of lunging. With minimal tack, just a rope halter and lead, send him out on a circle, but do lots and lots of inside turns. Start slowly, but if you can get him actively trotting around, and then rock back on his haunches to make the inside turn in one flowing movement, that does wonders for developing the back, and the back end.