I agree with Erica and Brianna. By doing this you are letting him clearly know what you expect him to do. Tell him ahead of time that this is the new routine. Tell him that you know this is difficult for him but you know he can learn how to stay in bed and go to sleep. Perhaps play up the big boy part a bit.
I suggest that by being solicitous, laying down with him, sitting in a room across the hall and what do you do when he gets up, might be giving him the message that you are concerned that he won't go to sleep by himself.
I think I would offer him the opportunity to have you lay down with him for a set period of time in exchange for him staying in bed when you leave. If he gets up once you leave then the next night you don't lay down with him. If he does stay in bed the next night then you'll lay down with him again. That may complicate the issue. Its an idea I haven't tried or heard used by anyone else.
Supernanny uses this method but she has the added step of you stay sitting down in the room with him or in the hallway so that you can immediately put him back in bed, without talking to him or making eye contact.
I do think it's important to have as little with your son as possible and that the action of putting him back to bed is quick. He will learn thru consistent repetition that he is to be in bed.