M.L.
No crazy rule! Just take him back to bed - not to punish him, but to congratulate and reward him for being a big boy. Going back to bed is what big boys do! Then he gets to play in the morning.
When the new baby comes, he can continue being a big boy by taking care of himself at night - which is more than the baby will be able to do. Poor baby! He will also be able to "help" during the day by bringing you things you need for the baby once in a while, and by having you read to him while the baby eats.