S.G.
This is pretty normal. That is why kids don't get glass plates. Be firm and tell him "no" when he throws the food or the cup. When he throws the cup, tell him it is dirty and sit it by the sink. If he throws food again, say "you must be done" and take the plate away. He will learn that he can't throw food and have it. If he is still hungry then you can feed him, it will make him mad but he is smart and he will know that if he wants to feed himself, he should use manners at the table. It will take a while but don't give up. My granddaughter does the same thing, only we have the added problem of having a dog who sits by her high chair and she thinks it is fun to throw food down to the dog. It is back firing on her because now if she has a cookie or something and holds it around the dog, the dog sees she gets her fair share, making my granddaughter really mad and she says in a stern voice "NO aira" Dog's name is Tara.