I ditto Jennifer.
I have 2 kids... both had different personalities and sleep patterns. What worked for my daughter would not have worked for my son, and visa versa. SO, as a Mom, I rolled with the punches (that is what a baby does- wake), and provided them with what they needed.
There is NO "rule" that a baby HAS TO sleep all night, by themselves, at this age.
MANY MANY kids, developmentally, do NOT sleep all night by themselves, until about 2 years old. Its normal.
There are 2 perspectives on this:
1) get a baby to sleep ALL night, by forcing it. Thus, letting them cry it out etc.
2) rolling with the punches and providing the baby with what they need, understanding that a baby DOES wake during the night... AND they go through all sorts of developmental changes ALL throughout childhood. This also means, understanding that their sleep patterns changes too, with each age set they become.
Now, per my Pediatrician, a baby needs to be fed ON demand for the first year of life. Breastmilk/Formula is their PRIMARY source of nutrition... NOT solids and NOT other liquids. They are growing very rapidly the first year, and developmentally, they NEED increased intake, to sustain their growth and nutritional requirements and caloric needs. If they do not get it, they cannot sleep.
It is NOT "creating a bad habit" if you go to him and comfort him. Your baby is simply changing. Its normal. At this age, they are teething, sitting up, rolling over, pulling-up, standing maybe... and these gross motor developments, tweak their sleep ability. It is a natural thing. AND yes, they do it in their sleep too. Its instinctive. You cannot just tell them don't do it. They are a baby.
If you don't want to cry it out, then don't. That is not the cure all for all babies. Not all babies are the same.
ALSO, at this age, per my Pediatrician, they are going through "separation anxiety." And YES, even when they are supposed to be sleeping. They look for you. They get lonely.
They are a baby.
ALL OF THESE THINGS, puts a kink into their usual sleep patterns. Even their REM sleep "ability" is changing.
I personally would not lock my baby in a room and let them cry it out.
I recommend the book: "What To Expect The First Year." You can find it at any bookstore or online like at Amazon.
Your baby is normal.
ALSO, make sure he is napping regularly. An "over-tired" baby or child, actually will NOT sleep well at night and will wake more. AND, make sure you are feeding on demand, 24/7, and at night, and ESPECIALLY when going through a growth-spurt. (9 months is a growth-spurt time and developmental time of changes).
Don't worry.
Mostly, the "expectations" a parents has about their baby HAVING TO sleep all night... is what causes frustration. Versus, going according to a baby's needs and normal developmental needs and changes. Expecting "more" than what a baby or child can do, will always lead to stress/frustration for the Parent. So expectations upon a baby/child, has to be "age-appropriate."
All the best,
Susan