The size of the horse's wither probably has less impact on saddle fitting than the shape of his back, both along his topline and across his shoulders. Those are the aspects I would look at first, and then make sure you will have adequate clearance for his wither, can accommodate his girth groove, etc.
As Peter, Paul, and Mary say, a dragon lives forever.
I have been looking at these! What model do you have? Can you give a short review?
Symphonie. It is a very plain saddle - moderate knee roll and deepish seat. I love the cut back pommel. It is comfy but not a couch. It is just a perfect moderate saddle that fits almost anything. Heavy, good quality and lasts forever. Mine is 15 years old and looks brand new.
No hour of life is wasted that is spent in the saddle. ~Winston Churchill
High withers often need a modified panel like a K or trapezius, and often a full front or wither gusset is a help, as is a deeper rear gusset to balance things out. A higher head tree can be a help as well. Something along the lines of an Albion SL or SLK (the former if the back is flat after the wither, and the latter if it's scoopy) high-head would be worth a shot; you can often find these used for a reasonable price.