Sounds like sleep walking to me too. I used to be a sleep walker as well, I could climb in and out of my top bunk bed with no problems! I could hold conversations and my sister even woke up once to my foot in her face, somehow I'd climbed into her bed up the other end. It was never a big problem, I never went to a Dr about it and I grew out of it by the time I was about 8 or 9.

If you are concerned that your DS is in danger then I would probably take him to a Dr to get checked out.