I see what you are talking about. If you look at the first and third photo you will see a sort of "bulge" just in front of the shoulder but not at the very outer/bottom edge of the neck. This is actually the vertebrae itself, not the muscle. Well, it's the vertebrae pushing outward , covered by muscle.
The neck has two curves (it's an S curve, right?) The lower curve and the upper. In your horse, the lower curve is a bit sharper and points a bit downward, so you can see it there, right?
However, it doesnt' seem to be that this is due to improper muscling, as a result of him bracing against the bit.
you can improve the upper neck muscles and work on getting him to lift at the base of the neck (akin to our lifting our collarbone up and back as we improve our posture)
Long and low work, having him back up and reallly ask him to lift (not above or behind the bit) his shoulders and lift his feet and PLACE them back, not drag them back.
He is a nice looking horse, really!