K.H.
The key is to be consistent - let him know that once it is time for bed, he must stay in his room. If he needs you, he has to stay in his room and call for you and you will come. When we switched my son to a bed, he would come out and try to see what we were doing. As others suggested, we would just march him back to bed and let him know that it is time for him to go to sleep and now he has to stay in bed. This worked after a few days.