My ds is like this (not so much anymore actually now I think about it, well the time of day has shifted :roll: ) - it used to be impossible for me to cook dinner because he'd want the food right there and then even if it was ages from dinner and if he'd only just eaten (I couldn't even go into the kitchen without him wanting food). These might not apply to your situation, but what I started to do was prepare a range of snacks before he decided he was really hungry to at least try and break the begging habit, get him engaged in that before going to the kitchen. If you know you want to go and cook dinner, prepare a part of dinner that he can nibble on while you're preparing the rest, I often would offer some raw tomato, carrot sticks etc and pop on a DVD (the only I would rely on it as a distraction) - do it while your ds is being positive and before the negativity sets in. That way he is still eating some of what I would like him to eat and if he is actually hungry, it's holding him off a little. Are you filling him up at each meal time? I find my ds is now really hungry from waking up in the morning until lunchtime - he basically keeps eating and soooo much! If there is a particular food that he's always wanting (my son does this too), I offer some of what he wants along with something else. If he keeps saying he wants some more of xx, I say that we've had enough of xx today and that too much of xx might make his tummy sore (I don't lie and say it's all gone because he knows it's not), and say if he's still hungry he can have either xx or xx (two choices that I'm happy with either way and I know that he likes). He knows now that if he is hungry he will pick one, if he's not then he generally gets bored of my consistent response and goes off and plays. This has been a long process for us though.
My other thought is, if he is really hungry, are you offering enough protein in his diet? This is a good energy food that lasts a long time. Sometimes a small chunk of cheese as a snack is really good. Or if you do peanut butter, some on a cracker or something.
HTH