My DS is the same, he's 3 and we still keep a baby gate on the door to pen him in. He is incessantly bouncing on the bed/opening drawers/talking but luckily his younger brother sleeps through it. We just firmly put him back in bed with the same phrases and go back to bed ourselves.
DD does this too, she is just 5. Knocks on our door two of three times a night with any old excuse (nail snag, worried about bike being left out in rain but it's not raining). Infuriating and exhausting as I can never get back to sleep. It has improved now she has started school as she is more tired, before that we did remove favourite priviliges though if it got too silly and use lots of praise when she did manage to stay in bed all night. I think they just get bored and decide to explore and test their independence