Having Down's Syndrome doesn't mean he can't be told what's not okay. People often draw a false correlation between innocence and intellectual disability. But individuals with special needs are often more than capable of testing limits to see whether they can get away with inappropriate behavior. What if he walked out of the store with an armload of snacks and didn't pay for them? Would you just shrug that off too, because Down's Syndrome? You're doing this guy no favors by giving him a pass until he gets arrested.

Down Syndrome doesn't always equal stupidity. You should have told him to leave OP. If he did it twice already he will keep doing it until you get it to stop.

Intervene and escort him out even if it's awkward, and see if you can put Safe Search on

As someone who works with people with disabilities I would have gone over to him told him that it is not appropriate what he is doing. He needed to stop and that, that is something you do in private. Also checking if there was a support worker or parent around that he may have wandered off from or trying to find a next of kin or someone. Next time if he comes in on his own make sure you say to him, that he needs to be respectful of other customers that are there.

I feel your pain. All you have to do to stop this is go up to them and let them know that this activity isn't okay and isn't allowed at your store. They should get the message. I know you don't want to go up to them because it's awkward but that's the easiest way I can think of to fix the situation.

I'd call the cops, like I would on anyone else. That is indecent exposure and if he is capable of using YouTube to find "****", he is capable of knowing that it is wrong to do in public.

Disconnect the Internet?

if he's a repeat offender I'd get in contact with the local authorities so whomever is supposed to be looking after him is aware, or if no one is, that someone is contacted about watching him from now on.

What he was doing was clearly inappropriate, and you could have told the other authorities.

You need a handbook to deal with a common sense situation?