Hi H.,
My twins did the samething when they were toddlers. What worked for us was just firmly telling them that it's not time to get up. We did not turn on the lights... or talk much since that would only cause them to be more alert. We didn't say anything to make them upset... like... if you don't go back to sleep you will lose _____________ as a punishment. Rather, we just firmly said to go back to sleep and as much as we love them.... we did not give a lot of hugs and kisses again at this time. (That would only reinforce the negative behavior.)
Again... we firmly... without raising our voices insisted that they stay in their beds. I would say... "(child's name), you NEED to stay in your bed and be quiet. It is not time to wake up yet." Sometimes... I would want to go into explaining why getting up in the middle of the night was not a good idea. But... my husband would stop me in my tracks and say..."Enough... J. they know the reason... leave their room." And at first that bothered me because I thought... well they deserve an explanation. But... his straight to the point way and that's that.... really worked best. (Sometimes less of anything is better... and less talking in this case was better.
Good luck and be consistent every night so your son knows that he cannot get his way with this. By being consistent, you are showing him that you are in control. (He can have control sometimes in other ways... but you all need your sleep.)
ALSO,, You may want to try putting him down to sleep a bit later... I am not sure if he is requiring a little less sleep now that he's not a baby. (They still need a lot, but not as much as a baby.)
J.