Have you unlocked the Sledge Hammer yet? The Endoskeleton room is located in the Showtime Stage section after breaking open the wall on the right side.
Not sure if you're still having trouble but I was in the same boat.
What I had initially missed was: In the room with the endoskeleton, there is actually a staircase just to the right of him. It's very faded so I missed seeing it initially. There's at least one key item here. Hope that helps!
