I just went through that with my son. He did that initially when he didn't like the food or was done eating. I first asked him to give it to me instead of throwing it on the floor, and I always told him that if he drops his food, he's done eating. I think he began to associate that with a way to communicate wanting to get down from the high chair. So when he gave the food to me instead of dropping it, I said "thank you" and got him down. When he threw it on the floor, I made him sit in the high chair for awhile (without food) before getting him down.
If he's not actually done eating, perhaps you could get him down and wait 15 minutes or so before seeing if he wants to eat more. At least he'll always associate that behavior with a consequence.
Also, I have a nephew who was premature and developed more slowly in the first couple of years of life. It took him well over two years to learn to talk, and he got very frustrated with not being able to communicate. He began acting out and hitting and biting his younger brother. My sister was in child development in college at the time, and her professor suggested that she teach her son sign language. The professor actually demonstrated it with my nephew, and got him to use the sign for "please" in order to get a snack from a vending machine. ("Do you want a snack? Show me what you want. Okay, say 'Please'" and she showed him the sign. She did this twice and then he mimicked her movement) My nephew picked up on simple signs quickly and the behavior problems disappeared, plus it taught him to talk more quickly.
It's just a suggestion. Here's a link if you are interested: http://www.signwithme.com/002_browse_signs.asp