I'll answer your question from two different perspectives: as someone who has a long history of working with kids, I'd say you've got yourself a "non" problem. You have a terrific eater who you could accomodate easily by just scheduling an hour for meals. Plan to eat your own food, let him linger at the table alone, if need be, and go take care of what you need to be doing. Or save/schedule little 10-15 minute tasks for this time, so he's not holding up your time/schedule. If you have a date to get to, start the meal earlier.
You do want to make sure there's nothing else on the table: no toys, books, pens and paper, etc. Just the food. And then, let him be, because IMO, he's getting way too much attention for a behavior you'd like to change. Some of these bargaining techniques are actually positive reinforcement for what he's doing. (Some aren't, and I'll address that in a second.)
Now, for the second perspective: I am a slow(er) eater. Always have been. My stepfather used to threaten the kids "Last one at the table does the dishes.".... well, I did a lot of dishes. I like eating more slowly, savoring the food I've made or received from someone else, and I like eating until I feel full, and then stopping. And eating in a more relaxed manner typically makes digestion easier on the body. I see all of these as good things. I am not a terribly slow eater, but I do know many family and friends who eat *way* too fast, which makes me look even slower. Lingering over a lovely dinner with friends and a glass of wine is just about one of the best things I can think of.
Please, too, stop taking away his possessions. You don't want him to develop eating disorders. Being forced to 'eat more quickly' in order to keep his favorite toys is seriously messing with a child's sense of security. If he begins to associate 'eat fast= keeping my favorite things', he'll be getting set up for some very unhealthy thinking regarding food/eating. If you need a "something to do" because you and your husband are getting fed up, start using a timer at meals. Set it for 50 minutes, and clear his space when the timer goes ding. This gives him plenty of time and you a sense of finality in regard to the meal. My son's preschool allows 45- one hour for lunch for the group, and these women have been doing this for a LONG time too.
Also know, too, that other social situations are going to correct this. At my preschool, the majority of kids don't take enough time for lunch-- I've had to set a timer to *keep* them at the table, because they want to go play, then are hungry an hour later. In your son's case, I'd just be cleaning up and let him eat during after-lunch storytime, and then ask him to clean up his lunch when we are getting ready to head outside a half-hour later. I also have a very slow eater in my group, too, and this is the option available to her. Her time at the table is slowly being whittled down, and this has been *her* work to do this, not mine. In kindergarten and in grade school, their daily structure/schedules will help him learn to keep moving along with lunch and to eat a bit faster. But see, too, how flexible you and your husband can be in accomodating an otherwise fine eater. Sorry this is so long. Hope you know how lucky you are that this is the big eating problem in your home!;)