I allowed DD to get out of the bed and grab books or whatever until the novelty of being able to do so wore off. BUT if she was crying, yelling, in distress, trying to get out of her room, ransacking the place to avoid falling asleep or anything like that....then I went in and put her back in bed. She was afraid of the dark so her door was open and she was VERY distressed. I had to sit in the hall and do a lot of WI/WO with her.
But, in general, I'd say if he is not upset/needing you or being unsafe, then let him be. IF it becomes a problem where he is not going to sleep for hours and it's really mucking up your whole routine and he's OT, etc. Then I'd go in and put him back into bed every time so he knows it's bedtime. I think they just have to learn how to handle their new found freedom.
It may also depend on how old your LO is and how verbal he is and how much he understands. You know him best and will be able to determine how much intervention he needs on your part.
Just my opinion.