Can you use a gate and then only do a verbal response from out of sight? That way it's less a game an he learns mommy and daddy are serious? Basically if you hear him get up, ignore it and leave him be until he goes to sleep, but if he calls our or cries for you just call back to him "It's time to sleep, go back to bed, honey." or something. This way he doesn't get the reward of your presence or your touch or anything, but still knows you are THERE in the house and as always doing your duty as a parent and making sure he's fine.
I have gotten to the point of telling my DD when she is really acting up about bedtime that it is time for sleep and she knows what she can do in her bed until she's sleepy. "You can read, sing, talk to your babydoll, but you may NOT call or scream for me. You have your water, your blanket, your paci, and you do NOT need anything else. You may read, sing, or talk until you're sleepy. I am NOT coming back up. End of story."
And when it has been a "game" she hasn't been too bad about staying up there until she drifts off. When she is OT or really needing me for something, she ends up crying and I have to go back. But it helps distinguish between her NEEDING me and her just WANTING me.