After having some one-on-one work with a new trainer, my horse has significantly improved. As stated in my initial post, there were no issues concerning saddle fit or health. While I did not have a chiro out, I did have an equine massage therapist, and eventually a vet, offer their opinion as to whether they thought something may be out of line. Fortunately they had nothing negative to point out.
The trainer who met with my horse and I offered a different approach to our problem. After working on getting him to yield his shoulders and focus on bending his rib cage in small circles, we moved up to lunging in smaller circles while focusing on different techniques that we didn’t think to use. In just three days of work we saw significant improvement as a result of a new approach and gentle, patient hand.