If it helps any, my little mare, who sounds perhaps similar, does well in a Hastilow jumping saddle, and as I understand it, they use the same trees as K&M and some of the Thorowgoods. (She’s actually a TB, but nobody believes me–she’s 15.2 and looks like a Morgan–built like a pear-shaped propane tank.) Her dressage saddle, after many failures, is a Frank Baines Reflex. Passier (Grand Gilbert) worked well when she was young.
For the little-wither, flatback types, I would suggest you get something with both front and rear gussets (wither and rear panel). That way the front can be reflocked to fit as his withers change (said little mare finally popped some withers in her…six year old year). The back can be reflocked to match the angle of his back away from his spine more easily if there are gussets back there. Otherwise you can’t do much with the shape.
