Ha! I have the opposite problem.... 3 bites; "I'm doooooone!"
If you are concerned, or it's bugging you, maybe set a timer? Determine that eating time is a certain number of minutes (be it 15 or 30 or whatever) and EVERYONE stays at the table for that long. If you're done, you wait politely and patiently, finish your drink or whatever. If you aren't done, too bad.
It sounds like mealtime has become too much of a social time for him. And I would just keep doing what you are doing, encouraging eating not chit-chat. A couple days of reminding him that he only has "until the beep" (on the stove timer or whatever you use) and him losing the plate before he is done should give him the message. And I would make sure parents know that that is the plan. If they seem to have an issue ("Well, why can't he take as long as he needs, it's not a big deal!") just remind them that you have other kids to take care of, get down for naps, and a routine to keep, etc.