K.R.
consider chiropractic
if his spine is out of alignment (guaranteed if he was medically assisted out of the womb, but can happen post-birth lots of ways too), he probably literally is in enough pain or discomfort not to sleep
(chiropractic for babies is feather-light, not at all the thumpy-bumpy stuff some adult chiropractors do)
or if you aren't a sleep-on-tummy mom (I didn't read the rest of the replies), maybe try one of those side-sleeper cushions
(the whole sleep-on-their-back thing, I don't hold with; I've seen very tidy explanations of why it's the chemicals in the matresses that make tummy-sleep a danger ... but if you are putting him on his back and he can't sleep that way, you already know you need to find some other solution)