Super interesting article from Today's Parent:
https://www.todaysparent.com/baby/behaviour/sleep-training?page=0,0
Basically says that what you believe about baby sleep influences how successful you are at CIO/sleep training. The study is on parents attitudes and baby's disposition and how it affects your success with sleep training.
Just a really interesting perspective. Basically I feel like maybe its not so odd that DS does NOT respond to any sleep training at all.