J.,
After reading the previous replies, I'm going to do my best to give you some answers. First, children are not usually tested for ADHD (or any other psychological issues) until they are in school and there are issues that have arised in school and school work. This usually occurs about 1st grade. Both of my sons were diagnosed w/ADHD in 1st grade, two separate school systems. My eldest nephew was dianosed in 3rd grade, while his younger brother was not diagnosed as having any developmental issues.
My daughter, however, has had some issues w/disappearing acts, which would occur in the middle of the night, while the rest of us were asleep. I had lived in an apartment and she would turn the doorknob, open the door, and go upstairs to the apartment of an elderly neighbor. I had no idea this was going on until about 3:00 am there was a loud knock at my door and there was my daughter w/my elderly upstairs neighbor. She said my daughter told her that she couldn't open her door. The doors automatically locked when they were shut. So what I did was I called my brother and I had him install a latching lock. This particular lock is quite easy to install: it was a hook (on the door) w/an eyelet (in the door frame). We placed it quite high, moved the living room furniture around, so my daughter couldn't reach it.
With all the above in mind, you might want to consider having such a lock installed. If you rent, you'll have to discuss installing such a lock w/your landlord. When you, make sure you mention that this is stricktly due to the safety issues of your son's random strolls. My friend's mom had a door handle installed on her kitchen door which you have to fiddle w/the door handle to open it. I'm not sure how much it costs, but what I did left no room for my daughter to figure it out. ;-)