My parents are in a similar situation.
I have a younger brother who is down syndrome, he's 13 and one of twins. I was just hearing a story tonight about how sneeky he is getting.
He loves crisps, he would eat them all day if he was allowed. In fact, he'll sneak down early in the morning and get a packet, and sit eating them in front of the tv.
My parents tried hiding them in the bread bin, but he found them, they tried hiding them somewhere else, but he still found them. So now they have locked them in the utility room. The lock is simply a hook at the top of the door where he can't reach.
This seemed to be working, until my Dad went down a couple of days ago and thought he could smell crisps on my brothers breath. Everytime my Dad walked in to the kitchen, my brother would give him a funny look.
It wasn't long before he found a packet of crisps hidden in the bread bin. I think he was probably sneaking a crisp every time my dad walked out
My Dad couldn't understand how he had got the crisps though. So he locked the utility room door, and said that he couldn't get in, and asked whether my brother could help, to which he signed "yes" (he can't speak much yet), got hold of the broom, and knocked the hook out of the hoop, unlocking the door
I think they are at a loss at what to do now. No matter how they try to keep the crisps away from him, he manages to get to them. The trouble is, he's overweight, and the crisps aren't helping, but they don't want to stop buying them for him altogether.