I have found that when I stay in the room with my grandchildren they take longer to go to sleep unless I'm lying down with them, unresponsive to their questions and/or comments.
Their mother puts on a CD that contains either soft music or a story and then she leaves the room. Sometimes the 5 yo, when he was younger, had difficulty falling asleep and she just quietly taken him back to bed. Either he's outgrown it, has adapted, or her putting him back to bed has taught him that staying awake does not get him attention.
My granddaughter, when she's at my house, will usually go to sleep while I'm reading to her or while listening to a story on CD.
I think that a big part of being able to go to sleep is having a night time routine. Perhaps a bath, quiet time, reading stories, or even watching a 30 minute quiet DVD such as the Little Bear series. I remember my mother rubbing my and my brothers' backs for a short period of time. She didn't stay in the room longer than 10 minutes or so.
Does he have a night light? That has helped my grandchildren too. When my granddaughter was 3 I frequently rocked her until she was sleepy.
My daughter, who came to me as a foster daughter at age 7, had difficulty getting to sleep. She frequently wanted to talk about things. If your son spends his day elsewhere, or away from you, 10-15 minutes letting him talk may help. I did learn with my grandchildren that if I spent more time than a few minutes after they were in bed they stayed awake longer.
I suggest that the adult's confidence that the child will go asleep if left alone might help. If your son has a quiet bed time routing I suggest trying after you stay with him a short period of time, reading, rubbing his back, or some other soothing routine that you tell him that you're confident that he will learn how to go to sleep before you leave the room. Empathize with his difficulty but reinforce that going to sleep is going to happen. Encourage him to not think about it. Put a CD on to help distract him from focusing on going to sleep.
My daughter became quite frustrated because her kids had difficulty going to sleep. When she stopped paying attention to it they gradually went to sleep more quickly.