it could be "delayed" crying, where hes crying about something from before he went to sleep? is he crying when he falls asleep? there was a recent blog post on crying :
http://www.askmoxie.org/2006/06/babies_and_cio.html
there are kinds of babies who might need to whimper for a short period (few minutes - less than 5 definatly less than 10 minutes) to release and sleep, while others just get more and more worked up. my son was an increaser, and its exhausting, but very fulfilling now that hes grown and sleep is a non-issue for us.
other thoughts could be teething. you could try to give some teething relief thing before baby goes to sleep.
other things like gas could also cause this issue. is he gassy? maybe pumping his legs as a routine before sleep is a good thing.
the other thing is that babies need their mamas even when they are sleeping. it might be that hes just sensing the distance or the absence of you and is missing you. remember that theres nothing wrong with listening to your instincts, even if that means that you dont separate your son into another room from you. babies regulate even their own breathing by hearing yours, so its sometimes vital for babies to self-regulate and relax by being near their mothers and fathers.
anyway, listen to your heart. what does your heart and instinct tell you to do? dont brush aside those thoughts, i dont care who tells you that they are rediculous, or that you shouldnt spoil your son (remember, food spoils when it is left alone or neglected and it rots, not when its paid proper attention to). just do whatever you know in your heart your son needs. you will NEVER regret this decision. i know i dont. sleep is a non issue for babies who sleep with/near extremely responsive parents. :) you dont have to have him in the same bed as you, but decreasing the space between you is going to help both him be comforted that you are near, and help you not to have to wander in the middle of the night to get to him.
anyway.good luck. just listen to your heart. its likely you know the answer deep down!