Quoting Ms.Eagles:" SO won't let me pick him up He says he "needs to learn to put himself to sleep" "this book says do ... [snip!] ... he isn't crying for as long (or as hard) as he did the first night, it still hurts my heart. Believe me, this is a last resort."
Tell your SO that your baby is in distress. CIO causes high blood pressure, bad changes in baby's brain, it messes up their intellectual, emotional, and social development, it's harmful to the parent/child relationship, and the only reason the baby eventually falls asleep is because they are so traumatized. There is really nothing good about CIO. It hardly ever works.
Trust me, I know how hard it is, my NINETEEN month old just started sleeping all night a month ago. But you don't need to torture him like that. And yes I really do see it as torture.