Brothers with Down Syndrome.

[Nice]

but what can we do? It's only when he is in that mood... I just don't like sitting beside him anymore...

actually that sounds really serious and dangerous. IMO you should try approaching him about that matter. Him throwing knives and forks at people is not exactly a small issue, even if it rarely happens.

I don't know much about autism and how things work related to it, but you should explain to him the consuqences of the things he does. If it was a regular kid i'd just say that the kid needs a harsh discipline but in your case I suggest that you come to him in a peaceful way.

Using the case where he threw a fork at you, come to him and explain what could have happened in the worst case scenario and what would happen after that and in general how its going to badly affect him and everyone else.

I'm just going to be really honest here, but in my opinion saying "What can we do, its the just way he is" is a very very bad way of handling the situation, unless you plan to have him isolated from the whole world for the rest of his life

EDIT: Bear in mind that I'm just explaining how I would deal with the situation this sure as hell not an expert advice xD It could be completely wrong but its what I think and with that said ---

About the glass dropping part and trying to get attention, at that point I'd make sure he gets punished good and proper so he gets taught that demanding attention is bad.

Autism or not, the best cure for a bad behaviour is always a sharp voice and discipline. As far as i know, gentle words or sitting back and justifying bad behaviour NEVER fixed anything