Part of the problem that we have is that his left shoulder actually extends about an inch farther back than his right. I can easily slide my hand down both shoulders with no resistance. His shoulders and back are really asymmetrical. Right now, the only area left to build up is on either side of his withers, and a bit behind that. The massuse, chiro, trainer, and saddle fitter are fairly confident we won't see any more drastic changes in his shoulders or back at his age. If he builds up in the final spot we are seeing new muscle, it will actually provide some more support for the front of the saddle, giving it a slight lift in front, and actually giving the shoulder a bit more room. I hope that makes sense!
I've spent most of my life riding horses. The rest I've just wasted.