Maybe try suggesting something like ok that he wakes up but he has to do something in bed not get up and play. As long as the child stays in bed he is more likely to fall asleep or at least his body is resting more. He is old enough to stay in his room if he wakes in the night, to have a gate at his door and his room childproofed so he can play in there unsupervised.

When did they move him from crib to bed or have they?