Two steps forward, One step back

Well... It was good while it lasted.
Potty time is still party time. Thank god for that. My upstairs is just starting to lose the scent of an overdue litter box.
The sleeping in bed.... shot to hell.
The last couple of nights, up he goes, sweet as can be, falls fast to sleep, then by 12:30AM he's ninja'd his way out of his room. Neither his father nor I hear a peep. We're still using the monitor, so I can't figure how we're both missing it. Of course the fact our bedroom is on the first floor, and his the 2nd probably doesn't help.
He's waking up and turning EVERY single light on in the house. The dog (bless his sainted soul) is waking Brent up (another reason I like him) Brent notices it's very light out in the house. (we keep our door partially open) When he goes to investigate... Ben's Room, the play room, the hall, the bathroom, the living room, the dining room, and the kitchen... All ablaze like we''re having a party. And there he sits by the computer. Damn! Not only does it wake us up, but it is a little scary.
Poor guy is obviously afraid of the dark. Brent tells him "Let's go, we're going back to bed" which gets him an obviously frightened "No Way!" And Mr. Softy himself leads him into our bedroom. Most of this I found out this morning, with the exception of his route to bed is right over my stomach. At 46lbs, I notice.
Poor guy! I'm not sure what to do. Even "typical" kids are hard to convince there's nothing to be afraid of. I seriously doubt he'd hear our words at all.
So... what to do? Do we get an external lock so he can't get out at night (at least without our knowing about it). That seems kind of mean. What to do???????