I would find a Dressage instructor and get some lessons. Dressage will help teach him how to effective use his hind end and his back, to help strength his topline and his ab muscles.
I would also find a good chiropractor, preferably one that also does acupuncture.
For tack, I would invest in a Skito Correction pad
. The memory foam will fill in for his sway back and lack of muscle, and allow him to build muscle as he's worked properly. You can swap out the shims as he improves. The foam will make him more comfortable, which will encourage him to use his back and abs more.