4 year won't stay in his bed

My 4 year grandson goes to bed without any problem. But he never stays in bed. I hear him sneaking out of his room and he doesn't get into anything he just comes into my room very quietly. After I ask if he is alright I then tell him to go back to bed. This can go on for up to 2 hours. I know he is afraid of the door being close so I make sure it is open but I'm thinking this might be my only solution.