I'm with the posts by Jeanie and Laeh. He is not old enough yet to manipulate you into staying in the room, as others seem to indicate. He simply wants to know you're there.
He's still young enough to think that when you leave the room, you disappear completely from the universe, in his mind; so he's distressed when you leave. He's close now to the age when he will realize you still exist somewhere else when you leave him -- but he's not quite there yet.
I don't think it's spoiling a child to go to him and meet his immediate emotional need for you at this point; it does not mean he will still be crying for you to be in the room when he's five! (Or even four, or even two.) If you gradually prepare for moving farther and farther away from him as he goes to sleep, he'll make the transition. When my daughter moved to a toddler bed, I used to sit in the hallway outside her door -- where I was outside the room but visible -- until she was asleep. After a while of doing that, I could then leave her, but checked in on her after five minutes; then after 10 more minutes; then after a longer wait of 15 minutes, and she eventually would be asleep before the next visit. But she knew I was around.
So many posts on Mamapedia are about "how do I get my child to fall asleep on his or her own, without me, without asking for me," but it does happen -- gradually and gently if you work it that way, which is better than screaming it out. If you are there for him, he learns trust and knows you'll meet his needs; that makes him more secure, and secure kids become more independent kids, I think. But if you leave him to cry it out, he learns that the person he most wants does not come when he feels insecure and distressed -- which can only teach him lack of trust and make him feel less, not more, secure.
For now, try moving physically farther from him but still be in the room; even turn yourself away from him so you're not facing him as he's in the crib; and then later perhaps move out the door or try returning at lengthening intervals. Don't respond too strongly to those first minutes of hard fussing -- you want him to know you're there but you're not going to necessarily pick him up or talk to him while he's fussing and settling, you're just present.