Oh, the freedom of a bed with no bars! That's the big difference now, J.. He has CHOICES and he is heady with the opportunity to exercise them. I know you don't want to punish him, and I wouldn't necessarily look at anything you do as punishment. This is a "teachable moment" because you have a chance to start teaching him about good and bad choices and the consequences that go with them. You get to start young, which is really what you want. The choices just get harder and the younger they start learning to weigh their options and make appropriate choices, the better.
You might start a bedtime routine if you don't have one. A bedtime snack, a bath before bed, bedtime pajamas, and a bedtime story. Notice bed is in all of the phrases. If you start the routine with the reinforcement that it's all about bed, he will have a chance to gear himself toward bedtime. I like the idea that he may have a stuffed animal as a sleeping buddy, but if he gets up more than twice he loses it. I would also do a reward for when he stays in his bed and falls asleep. That way he could see the difference between good choices and good consequences and poor choices and bad consequences. A sticker chart might be helpful, too.
Hope this helps.
L.