E.B.
It could be a nutritional deficiency, or any number of things, from a medical condition to just a really active hungry boy with less than desirable table manners.
I would begin making a notebook of a couple of things: what time he's eating, what he has been doing (has he been playing really hard or just sitting watching tv prior to eating), and what foods he is eating (be specific: don't just say "spaghetti", say "spaghetti with ... and then specify the kind of sauce, and the basic ingredients or brand of sauce). If you can, video him on your phone or camera when he's cramming food in and stealing from his brothers. Don't make a big deal of the filming, do it in a subtle way.
What kind of food is he eating? Do you cook healthy meals from scratch, are you trying to cut calories, do you cook mostly from mixes like Hamburger Helper, etc? Does he eat snacks frequently or is he to wait until the family sits at the table for mealtime?
If this occurs at nearly every meal and happens nearly every day, I would ask your doctor, but not before you have some solid info to refer to in the form of a log or journal. If you do journal it, you may see some clues yourself.