Your problem may not only be the top of the gullet hitting his withers but your saddle is also pinching the side of his withers - as indicated by the white hairs. A pad is not going to help since you really need to replace your saddle.
An "A" fork saddle will generally have more gullet clearance then a saddle with a regular fork. Gaited saddles have more flair in the front of the bars but don't give more gullet clearance.
BTW, you have an excellent saddle fitter pretty close to you. Kevin from Southern Trails Saddle, who is also a member here.