Are you seeing that too? I was hoping that it was just because he has like nothing on his withers or spine right now and that as he would fill out the saddle would start to fit better.
Not only that, but a bridging or in other way not properly fitting saddle will keep the horse from filling out as well since it puts too much point-preassure for the horse to be able to arch the back and use the back muscles properly.
If it presses down in the front, the horse can/usually get ''holes'' there where muscles should be. Sadly, that's so common that many people think that's a normal and healthy thing that comes when the horse mature, and are just happy that it makes the saddle sit steadier... :3 *off topic info*