my boy fell from his playpen when he's about 7 months old. i thought he just fell asleep, so i went upstairs just for about 5 mins or so. unfortunately he woke up, stood up and try to catch the musical mobile thing above the playpen. i heard a big "boom". when i went downstairs, he's on the floor crying and the musical mobile thing fell apart.
that's the first time that he fell, so i never forgot. thanks God that he fell onto the carpet so he's fine.
after that accident, and when he's a bit older, he's doing the same thing yr son's doing. so i seldom put him into the playpen since then, especially when no body's around watching him.