Has your son seen a doctor about this?
Has he had any PT for the situation?
Some may be improved by improving his position when sitting in class & standing.
Episode 351: The One Joint Rule
Shoulder position can be improved by addressing the t spine as well.
Basic T-Spine Mobilization
T-Spine Smash – Double Lacrosse Ball
T-Spine Plate Smash
T-Spine Overhead Barbell Smash
T-Spine Internal Rotation Smash