H.S.
You can reward him for staying in his bed all night - little trinkets or whatever. Or, get that baby gate out (a tall one) and put it on his bedroom door. :) We have this and a baby monitor in the room so that if our dd really needs us, she can just talk to us and we'll come get her to go to the bathroom or whatever. We also have a baby lock on our bedroom door and sleep with it shut.
Good luck.
ps. our dd is 4
H.