My first bit of advice is to get him some vitamins and in a hurry, if he hasn't eaten in weeks.
Next, he's very delayed in his speech but he's not delayed in his understanding, right? This should not limit your communication. If he can hear and understand...that's enough. Ask him what he wants by maybe, going through some choices. For everything he says yes or nods his head for...good. If he nods his head or says yes for sweets or foods that's not really good for him, tell he that he can get those things but he first, has to eat his regular foods. I bet he'll understand that.
I have a 4 year old, soon to be 5...tomorrow, as a matter of fact and he has a speech impairment based on his articulation. That don't stop nothing! We communicate well. He'll tell me he wants fruit snacks, soda, cake, cupcakes, cookies, strawberry shortcake, ice cream, popicicles, etc. "Sure", I tell him "but you're gonna have to eat your solid foods first." He gets mad and pouts...I get mad and pout, too. Then he'll come back and look at me, point and I'll tell him, "I don't understand sign language." He use to point and I'd ask him if the cat had his tongue. He's be a smart a _ _ and nod as to say yes. I'd be a smart a _ _ and tell him that he'd better hurry up and find that cat and get his tongue back. It's kinda hard to understand what he's saying sometimes but I make him talk. If it's something he wants, I tell him to open him mouth and say it. Now if I still didn't understand...next step. He'd take me to it.
Now if that's the only way your son knows how to tell you something, fine. Oh yes you CAN threaten him! When he points at sh _ _ he doesn't need, you point at sh _ _ he does need. Tell him he can have that stuff if he'll eat the stuff you want him to eat first, and explain to him why he needs to eat those things. Tell him they're not only good for him, but they will make him strong and he'll grow up to be a big boy. Does he like to watch WWF? If he does, tell him he'll grow up to be big and strong like the Undertaker or John Ciena, or even like his daddy. You gotta make it sound good.