Dear C.,
My youngest son was out of the bed and into a lower bunk bed at the same age as your son is now. Bed time at night wasn't the problem, it was nap time. If he had even his little finger moving (I kid you not) he would not nap. And come evening time he was a terror. So this is what worked for me. I would watch for the "I'm tired" signs, then I would say "nap time" and for the first week drag him kicking and screaming to his room, put him on the lower bunk and get in with him, wrap my arms around him, and hold him still. I know it sounds cruel, but I would hold him still, even his little hands. After about 10 minites the first day he was asleep, the second day 8 minites, by the fifth day it was three minites. Sometimes I was so tired I'd fall asleep with him but I didn't care at that point. Boys can be so active and curious that they will be afraid to go to sleep because they might miss something, unlike girls who will dutifully take a nap, but come bedtime it's nothing but endless drama. By the end of the second week he figured out that nap time was inevitable, and that he felt better taking a nap than not. By the third week he would just come tell me he was tired, I only had to sit with him for a few minites and he'd be out, at the end of a month he went to his room on his own climbed in and went to sleep.
I would suggest that you not lock him in his room, because much like a prisoner who is locked in, he will destroy his environment because there is nothing else left to do. And you don't want him to equate his room with a punishment. They are very smart and pick up on that.
Well that's just my 2 cents worth, and it did work for me, you might give it a shot.
God bless