Does he just get up because he doesn't like sleeping in his own bed? if no I would just take him by the hand and keep putting back in his bed until he stays there no matter if he cries or not. But first make sure something it isn't something that is making him uncomfortable. My daughter used to do this because it would be too hot for some reason in her room it gets really hot summer and winter. I turned on the ceiling fan and kept the door open so it stayed cool and she's been in her own bed ever since.
It was about that time that I had to stop giving my son a nap during the day. and for like 3 months or so it was hard because the end of the day was rough because he was tired but He would go right to sleep and sleep all night (7pm to 7am). The other thing you could try is a baby gate at his door. Leave the door open but the gate closed. He may fall asleep on the floor a few times but eventually just getting up will not be as "rewarding" for him. Hope this helps.
Does he go to sleep well at first? If he just gets out of bed in the middle of the night I've read that you should direct the child back to bed without speaking to him or engaging in in any way. The show "Raising Sextuplets" had a problem with this and an expert suggested that they lock the bedroom door. If their bedroom is perfectly safe and childproofed, there is no reason you can't lock the door and her reasoning was that the kids will just get board and realize they are really still tired and go back to sleep. Learning to go back to sleep after a potty break or drink of water takes a while to do.