I was about 13/14 when my parents let us roam around.
We all went to the same park or they knew which park we were at (this was pre AK & some trips may have been pre-MGM).
It was me, my cousin who was my age but more of a fraidy cat to move alone & his 2 twin cousins who were a year younger then us.
Our parents gave us money, our ticket (if they were eating out of the park first) & told us where to meet them & at what time (roughly).
It was great. Many nights we would watch Illuminations & ask our parents if we could go to the MK after. They always said yes. So we'd high tail it to the monorail & then go to MK til midinight/1am (depending on what time it closed).
Most trips we were at the Poly so we didn't have to worry about the buses & just hopped on the monorail home. Plus they knew we knew where we were going.
In today's age, I would just give him a cell phone & be in contact with him & know where he is going. But at the same time I would make him spend alot of the time with you since it is a family vacation.