Other than being impolite, the main problem here is that a child can choke with too much in his mouth. Until your son learns to regulate his behavior, give him only enough food at one time for a mouthful. If a sandwich is on the table, cut it into bite-sized pieces and give him one at a time. If pasta is on the menu, dole out small amounts. If he asks for more at once, let him know that you will do that only if he chews and swallows before putting more in.
Are mealtimes conversation times in the family? They should be. Include him in the conversation and make the point that he cannot participate when his mouth is too full.