Hi E.,
I haven't experienced this with my child, but my sister walked in her sleep. She is 12 years younger then I am... so I remember alot from when she was little.
I remember she started about the same age as your son. We would find her all over the house. My mom would go into her room in the middle of the night to check on her and she would be gone. The first time it happened it really scared her. Anyway, after a 'trying to escape the house' incident my dad put a chain lock on all the doors, up high enough she couldn't reach it.
I will say, we could always tell she was sleep walking because we would ask her questions and her answers wouldn't make any sense. Once I was babysitting and she wandered out into the living room, I asked her what she was doing and she said 'I wanted to go to the light'. Which on some level makes sense...she saw the living room light and went there. If you could see her eyes.... it was like that old saying 'nice house, but nobody home'. She would have this glazy distant look.... you could tell things just weren't registering.
Basically what happens during sleep walking is this... when we get into our dream state our bodies produce a chemical that basically paralyzes us. When this chemical is not released in high enough quantities then we act out our dreams. I have not looked into it enough to know why the chemical is not being produced, but I'm sure you could find that out easily with a quick google search. My sister was more prone to sleep walking when she was super super super tired.
THe older my sister got the less the sleep walking occurred. I cant' tell you if she ever really outgrew it because at the age of 18 she was in a 4-wheeler accident and is not paralyzed from the chest down. I did go to school with a girl who walked in her sleep, and to my knowledge she didn't outgrow it, but I know others that did.
At this point, I would suggest putting locks on your outside doors to ensure your son can't get out in the night and just be prepared for bizzare happenings. My sister once tried to pee in the cupboard under the sink. She basically took a wrong turn on her way to the bathroom, wasn't awake, and tried to pee in the cupboard. THankfully my mom was awake at the kitchen table and got her into the bathroom.
When she was older and wanted to go on an overnight to someones house or with a group, my mom would just be sure to tell the person/people supervising of the issue and they would be sure to take precautions. Once she went on an overnight with the girlscouts. They put my sister in the very back of the tent so if she started to sleep walk she would have to walk over everyone else to get out of the tent. We never had any problems.
Honestly, at this point I wouldn't worry about it too much.