I know this thread is a bit old, but as someone else said, saddle fit with this guy will be critical, for his comfort and yours. You do not want to feel like you are sitting in a hammock. I would find the very best saddle fitter around and see if you cannot get a good saddle and pad system worked out for him. As my Arab aged, I used a keyhole shaped cutout to elevate the saddle in the back and keep it from dipping into his back. It was something my friend and I rigged up, but it was also before saddle fitting was really in vogue like it is now.
I would love to see a lightweight, synthetic maybe, english type saddle on this guy with more support under the seat area to give it and you proper position. IME, it is easier to fit english style saddles to a swaybacked horse. With a western saddle, I always had the problem of "bridging" over the sway, putting a lot of pressure on either end rather than even pressure along the whole saddle length. It is also hard to pad support for specific areas under a western saddle. Don't get me wrong, I ride western right now, but I do not currently have a swaybacked horse.
BTW, from the limits of that pic, that horse otherwise has pretty nice conformation and appears to be well bred in general. Once he sheds out, muscles up, and gets some more groceries, you will have a really nice looking horse. If he does not shed out well this spring, it could be a sign of Equine Cushing disease, very common in older horses. Have him body clipped, and have a chat with your vet about options for treatment.
Just some food for thought.