tie him down? LOL my son travels during the night a lot and sometimes i find him asleep on his floor. i dont stress it. is he coming to your bed? and you dont want that ? you will have to just be consistant in putting him back in his own bed.
try putting a fan in his room. sometimes kids don't like the silence at night. it can be creepy and they hear all the noises that the house makes and stuff. put that fan in there and he won't hear anything but white noise. and i'd turn it on high and point it away from him when it's cold. just a suggestion! good luck