Your boy is staying as close to you as possible, for safety's sake, in his little mind. When one of my four children expressed bedtime fear, I would lie down next to that little one until he or she fell asleep. Night after night I would do this, ( it only takes a few minutes out of your life, really, and they are little for such a short time,) until they could fall asleep on their own.
I would work up to that, saying, "I'll be back in five minutes, I just want to check on ..." I would come back in five minutes, again and again, until my little one fell asleep. Be sure to let him shut his door, if that makes him feel better, but go into his room and do something fun with him, at some point during the day. At night, be sure to use a good night light!
Remember, he won't go to college asking for you to sleep with him, so don't be afraid when others tell you that you are setting him up for disaster, loss of independence, ability to rely on himself, etc. You are NOT! You are meeting his need for safety and security.
Eventually he will feel safe, and grow up just fine, and you will be glad you comforted him when you look back on his sweet babyhood!
Love,
L. M