Ah, yes- this is a very common stage and the duration varies from child to child. Don't panic, it might not be so extreme with your son:)
The ladies here have already given great advice- so I will just briefly explain how we got through it (Emma is now 20 mos. Expecting Claire to arrive late July:).
Emma was not a chucker/launcher like your daughter, she would do the "umpire," wave her hands palm down back and forth across her tray like she was motioning that the batter was "SAFE!" Food (and dishes) would fly. We stopped giving her a plate and instead prepared her meal and sat it on the table with our food, and would only give her one or two bites to feed herself at a time. Food would still end up on the floor, but more would be eaten.
We also enlisted her help in the cleanup, after eating I would get down on the floor with her and explain to her that we needed to pick the food up- I would help her find pieces to put back onto her plate ( I did most of the work, but she got the idea).
What you feed them during this stage makes a big difference too, of course you don't want to comprimise their nutrition, but you might want to keep things like ketchup off the menu for a while.
I can't remember how long this went on, but it did pass, and meals are much more enjoyable now:)
Good luck!