Like others here (right on, Gamma G), I don't believe in CIO. He's an infant and is utterly unable, developmentally, to do the reasoning that "If I cry she will come! I cried and she came! I got what I wanted and will do it next time." That is the reasoning that some folks think infants can do -- but they can't. (Not saying that YOU feel this way, just that it's an argument you'll hear from some people.)
An infant left to cry is not trying to manipulate the adults; the infant is in a state of pure isolation, and only knows that if you are not right there, you no longer exist. Remember, for quite a long time, babies think you disappear forever, every time you leave the room. So an infant left to CIO is being given the message that the adult cannot be depended on to be there to meet the child's needs -- and right now your son is pure need, all the time. If you think about it that way, do you see how CIO at this age, or even in a few months or a year, is so hard on a child who believes he's been abandoned because he has no idea you're in the next room?
So please don't worry about developing habits yet, etc. If he needs to co-sleep and YOU get better rest and less stress that way, do it (safely! Learn about things like pillows, blankets, suffocation risks in an adult bed, etc. before you do it). I did not ever do it but I did something else we came up with: I would sit on the floor next to my daughter's crib and keep one hand gently but firmly on her. She could see me and feel my touch but I didn't pick her up and put her down repeatedly and didn't talk to her after lights out. It worked for her; she was comforted and knew I was there and would go to sleep. Gradually over time I could just be in the room, and then outside the door, and then -- we were done and she was able to get to sleep on her own. This might be tougher for you since you have another child but if the kids have separate rooms, maybe your husband can take over bedtime for your older child while you do this with your infant.
But at your baby's age, remember, he does not have "habits" yet -- only needs to be met. And his needs are great -- teeth coming in, tiredness from crawling, etc.