Ah but you're obviously not as good a parent as the PP! Only those who 'hover' are good parents!!!
As for the 'they can't see anything' folks...I can tell you from my own personal experience that they CAN see things...especially if they're looking! A few years ago I was in a stall at MK and needed to deal with a *ahem* time of the month issue. I was in the middle of doing so when I heard a loud 'uh yuck!' comment and looked up to see an eye peering at me through the crack in the toilet door. I admit I swore loudly at the person involved - who appeared to be an 11 or 12 year old boy. I finsished what I was doing quickly and opened the door to see a mother ushering this lad out of the toilets. I shouted to her but she virtually ran out of the door to get away as quickly as possible. I would have followed but I needed to wash my hands.
Now my DD will be 11 on this trip and has recently started her periods...whilst I was angry about my incident my DD would be beyond mortified if this was to happen to her with a boy of her own age...can you even imagine?!
I'm with Hannathy on this one - if your son is too old to go in the stall with you then he stays outside or uses the mens room!
To the OP - having said all of the above I work with people with special needs and completely understand your situation but your son is, imho, a completely different scenario from the one I described above...if I were to see a young man, flicking a peice of paper being escorted into the handicapped stall of the women's restroom by his mother I wouldn't bat an eyelid.