Yeah, our dog helps us out a lot, too...I highly recommend the method. 
But, I disagree with getting him down. Do you really want to teach him that throwing food equals done? Because until he learns a new way to tell you, he'll be throwing food when he's done. Even for a 1yo, I spend a lot of time helping them to do what I'd prefer instead of just saying "no." Or getting them down or whatever.
I might have a conversation like this with my 1yo:
"Are you all done?" (make sign). Pick up the food and put it back on the tray. If she makes a move to throw it, I'd gently stop her hand. Repeat, "Are you all done? Say, 'all done'." (make sign). THEN get her down. I insert whatever word I think is what she's trying to tell me. If I get her down and she gets upset, I say, "Do you want more?" (make sign). I say "more?" several times, with the sign, and pick her up and put her in her seat. "Here's more." (Give her a few bites and repeat the sign).
But, I disagree with getting him down. Do you really want to teach him that throwing food equals done? Because until he learns a new way to tell you, he'll be throwing food when he's done. Even for a 1yo, I spend a lot of time helping them to do what I'd prefer instead of just saying "no." Or getting them down or whatever.
I might have a conversation like this with my 1yo:
"Are you all done?" (make sign). Pick up the food and put it back on the tray. If she makes a move to throw it, I'd gently stop her hand. Repeat, "Are you all done? Say, 'all done'." (make sign). THEN get her down. I insert whatever word I think is what she's trying to tell me. If I get her down and she gets upset, I say, "Do you want more?" (make sign). I say "more?" several times, with the sign, and pick her up and put her in her seat. "Here's more." (Give her a few bites and repeat the sign).
