J.F.
My son is the same way. He has two "robot friends" that have recently moved in with us. We set aside a place for them in the living room with their pillows and blankets (from the spares we keep in the linen closet for guests).
Each night we tuck them in, brush our teeth, then go read story and lights out. My husband and I go to our room shortly thereafter. (We don't sleep that early, but usually fold laundry, watch tv, etc). We frequently have to remind him a half hour later that it's bed time, but we're slowly making bedtime five minutes earlier each night, and this is helping as days go by. After only two weeks, we're noticing a huge improvement. He's not being naughty or anything, he just has an active imagination, which you can't click off like you can click off the light. :)
If it progresses and is starting to affect his performance at school, his attitude, appetite or things like that, talk to his pediatrician, it could be medical; but right now it sounds like he's just a kid with a great imagination. (which is a good thing) :) Good luck! Let us know how things turn out!