V.D.
This might sound crazy but it helped my friend years ago.
She put up a baby gate at the childs door and sometimes would find her asleep on the floor by the gate in the morning but at least everyone got a good nights sleep!
I also remember reading in a book that another option is taking down the regular door and replacing it with a screen door, that way the child can see out and be heard but cannot leave the room. You may have to go into the room a few times but make it clear that you are there and can still hear him but that it is time for him to stay in his own room.
Something I used when my daughter was 3 and didn't want to stay in her room was we bought glow in the dark stars and a flashlight. We put them on the ceiling and walls above and next to her bed and she loved to watch them fade out and then "repower" them with her flashlight.
Pretty soon she would drift off and not use the flashlight.
She also enjoyed music boxes...
Also, both of my children loved to sleep with a lot of stuffed animals.