That can be a volatile age socially and emotionally. Do you have any books that talk about common developmental changes that come at different times for children? T. Berry Brazelton's Touchpoints 3-6 might help you understand where your son is coming from. Marguerite Kelly is another good author on child development.
My advice is to respect his fears and need for you at night. I would not close the door on a distressed child at night. I think it's normal for preschool-aged children to go through phases of being able to sleep on their own and needing support from parents at night.
My parents had a policy of never letting children into their bed, and it didn't make me a great sleeper. I had terrible sleep problems even throughout elementary school. For that reason, I decided to try to meet my children's nighttime needs and not to make them tough it out on their own.