A couple of options come to mind for this. If he does this at a point in the meal when he is probably still hungry, you could tell him "No, that's not what we do with our food." and if he continues, immediately (very abruptly)take away the food and get him out of his chair and say something like "ok, I guess you are done." Then just walk away from him or put him in a time out. Then after a few minutes, ask if he would like to finish his meal. As you put him in his seat again you could say something like "ok, we are going to try this again, but the food stays in your
mouth." Don't hesitate to talk in adult words and sentences. Kids at this age understand a lot more than we think, and a lot more than they can say. Another option, that may not be as popular depending on your discipline style, is after warning him to stop, flick his mouth. Not real hard, just hard enough to sting a bit, while telling him to stop and keep the food in his mouth. He will learn pretty fast that this is unacceptable behavior and he will move on to something else.:-) Hope this helps.