I agree with Eliyah. Don't lock him in his room.
The technique mentioned by Eliyah is the one we used, we learned it from watching Super Nanny. It will take consistency on your part, and yes you may be tired for a few days until he gets it. The first time he gets out of his big boy bed, you take him back in there, give him hugs & kisses and say "good night". The second time he gets out of bed, you take him back to bed and only say "good night" - don't say anything else. The third and any additional times he gets out of bed, you take him back to bed and say NOTHING! This could take some time, but it will work. The first night will be the worst, then each night will get better and then one night he will stay in bed. What is happening right now is that you are giving him a reason to be up, by giving him his pacifier or milk (which by the way is horribly bad for his teeth). It is very unlikely that he really needs a drink or anything else he can come up with when he gets up, it's just an excuse to be out of bed. If you use this technique, you will all be getting better sleep within a few nights.
Good luck!