C.W.
I had the same problem with my little boy at that age. I would give him a few pieces of food at a time, not the whole plate at once. When he would drop the food over the edge he would look at me to see of I was watching. I would squeeze his hand and tell him, "Food stays on the table", in a stern voice so he would know that I was not playing. He stopped when he noticed that I would not stand for it. The hand squeeze is not too hard just enough to get his attention. It worked for us. Good luck!