K.,
This is SO normal...now, that your son has become more mobile and is able to see his world from different angles, he is becoming more aware of what is or is not around him. At this stage, my son was still off the one feeding a night in a breast milk/formula combo in a bottle...but, the seperation anxiety he began to experience threw a monkey wrench in our two months of solid routine. He was also, growing and the spurt caused the need for the re-addition of the nightly feed, which his Pediatrician said was totally normal. In his almost exact words, 'do what you gotta do'.
So, I too ended up rocking and singing to my son at night to get him to sleep. Really this phase in my opinion is all about helping the infant understand that it's okay that Mommy isn't always in your line of sight, and no matter what we'll always be back for nuturing and love. Infants in this phase, don't know that when they can't see you that you haven't just simply disappeared for good...so, there is a lot of insecurity that leads to crying and of course lack of sleep at night.
For us, it only took a couple of weeks to get him back into his pattern of knowing Mommy would come back if he needed something. They just need reassurance at this phase, and comfort through it.
Also, during the day I would play games like peek-a-boo to get him used to me leaving but, coming back. And, making him a part of my activities...like laundry and cooking. Today at 2.5 he's the best sock sorter!
I would rock my son to sleep, while playing a lullaby CD and telling him Mommy was going to stay with him until he fell asleep, but that I would always be back if he needed me. Since I got so used to him waking I would start to come at every noise and sound...most noises were him just resettling, so before you jump to comfort make sure it's not him just adjusting his comfort. But, I would come if he needed diaper change, feeding or some comfort back to sleep and then tell him the same thing...I'll be back...it worked over a short period of time, and he started to sleep through 'most' of the night.
I thinking sleeping through the night is one of those things that never really happens...not, even for me as an adult...but, I've got friends who insist that their kids sleep through the night. So, maybe but, I could never let my son CIO and leave him to figure out how to get back to sleep on his own...just me, though.
I hope some of this rambling helps!