C.S.
My kids went through this phase, & I just explained to them before bed, that they can get up when they see sunlight coming in their windows, if there is no sunlight then it is sleep time and the only reason to get up during sleep time is to go potty, get a drink, or if they are getting sick; otherwise it is just disturbing the rest of the family & that has consequences. Then explain your consequence, which should be his currency item. for my son it is his t.v. time or games. My daughter is her dancing.
Let him know that if he disturbs the rest of the family at night again, then he will not get to blank the next day and follow through the next day. Do not say anything that night cause then you'll be dealing with a fit, but just take him back to bed then the next day when he asks for blank, say, "Oh, I'm sorry, i would love to let you, but you chose to disturb the rest of the family last night, but maybe tomorrow you can, if you choose better tonight. K buddy? Then walk away & let him process.....If after a few days it still isn't getting through, add more consequences & continue the same dialog at night & in the morning. He's a smart kid & I'm sure he'll figure it out. I mean this is pretty normal, he has lost some of your attention & he's not ready for that yet, so he's getting his one on one time at night with you. Pretty smart really, but it will take it's toll on the family. I'd also try & give him at least 30 minutes of one on one time during the day, say during baby's nap. your in a transition period & he just needs guideance on how to get through it. I have found talking & communicating are great, but follow through is the key. Let him know flat out why you believe he's doing this and that while you love spending time with him alone, you don't care to do it during sleep time. This validates that you do understand him and you still see him & that you do care about what he needs & wants. Just assure him that he's still a priority, but at night, not gonna happen. Maybe explain too that the baby will not be in your room forever, just until the baby is old enough to sleep by herself. Then she too will sleep in her own room when she is a big girl like he is a big boy. I'm sure he will feel better about the whole situation & when he wakes at night, he'll make the right choice. Hope this helps you.