My son was a preemie baby and had to be put on Enfamil Gentleease so putting him to sleep at night I…

My son was a preemie baby and had to be put on Enfamil Gentleease so putting him to sleep at night I always had to rock him in my arms. He's now 6.5 months old and the only time I usually don't have to rock his is when he goes to bed at night, which he has to fall asleep in mine or my mom's lap, but if he wakes up in the middle of the night he has to be rocked back. I love my son, but he's already 17.4 pounds and it's taking a toll on my back, I can't imagine having to rock him still in the months to come. Has any other mom's had this problem before and helped it other then the cry it out method? I've tried leaving him in his crib but he crawls, sits up and pulls himself up on the crib already so he wouldn't just lay there and fall asleep.