DIY posture monitor

We noticed my son's uneven shoulders since last year. Turns out he has the scoliosis with a 19 degree curve from the mid back, as you can see from the Xray picture. There're other posture monitors(both DIY and commercial), but they all focus on leaning forward. That is not what we need. Since this project looks doable with the things/knowledge that I have, I created this monitor in about 3 days with some trial/error. His right shoulder is lower, so I allow it has more tolerance on the left than