Hi James,
I've worked "front line" in mental health/learning difficultity field most of my working life and know how difficult it can be, especially for the parents. However I can totally understand your reaction and would more than likely reacted the same way. I cannot understand why he would be allowed to have a glass and be at risk to himself as well as others.
I would be the first to argue that "everyone" regardless of ability has the right to live as normal a life as possible, but with that right comes responsibilities to other members of the community, unfortunately from experience I have found that is seldom the case.