Thanks everyone for the advice!
He is in a wide tree saddle. I've been working closely with a saddle fitter in my area to make sure his saddle stays appropriately fitted because when he came to me a year ago he had MAJOR atrophy from ill fitting saddles due to previous owners. He was also out at his withers and suffered from a lot of back pain. The muscle is building up, he looks significantly better around his withers than he did, it's just moving slower.
But as long as he is continuing to build the muscle up in that area should that rule out atrophy? I will definitely ask her to reevaluate the saddle fit! From the best of my knowledge though, it seems to fit him great. He is no longer upset about getting tacked up or riding (which he was when I first got him. He associated the saddle with back pain/pinching I am assuming).
And just for a few more pictures since I love him! (and his shark fin withers) Thanks bhorselover :) I think he is pretty adorable myself. He is great. I have a relationship with him unlike any other horse I've worked with in the past. He loves my niece too haha 165407_10152688919020058_184238532_n.jpg 602005_10152651154105058_346802324_n.jpg 20793_10152688975675058_1661482119_n.jpg