Good table manners
Small meals and snacks throughout the day are fine with me, but I sure like the idea of a sit down dinner for the family. So I'm curious what unschooling families do -- skip big sit down big meals, have them completely optional, etc.
It depends what your goals are.
If your goal is bodies present at the dinner table then the way you go about it will be different than if your goal is the family joyfully at the dinner table.
If having all bodies present is the goal, then some acceptable tools to achieve that are to make presence a requirement and to make sure they're very hungry when dinner is ready.
If having them happy is the goal, then those techniques won't work as well. If the tool you use is to obligate them to be at the table, then they fulfill that by occupying a seat at the table until excused while they think of all the things they'd rather be doing. If hunger is the tool you use to get them at the table then the atmosphere of dinner is about eating and not about being together.
For people to be happy, being at the table needs to be a choice. And if you want them to choose coming to the table over some other activity, dinner time needs to be a time people look forward to gathering together. You can feed them when they're hungry and ask if they'd like to draw or play GameBoy or have dessert or something else while they visit with the rest of the family at dinner time. Or dinner can be like show and tell where everyone informs dad of what went on that day. Or you can tell them having the family together is something that you really appreciate. (As a way of sharing your feelings rather than a way of guilting them into doing something you want them to do.) Or for really busy families, making one night a week a special dinner-together time (with candlelight :-). Or something that everyone looks forward to doing together.
It's not that no one can create a Norman Rockwell dinner with hunger and requirements to be there but that those aren't the cause of the atmosphere. The atmosphere isn't created by force. The atmosphere is created because of some other dynamic that makes people enjoy being together.
He can't stay still at the table with minimal manners.
Don't make him sit at the table!
When there are multiple goals for an activity, the goals tend to interfere with each other.
From what you've described it seems the unconscious goals of dinner time for you are: eating, learning proper etiquette, and family time.
I think it makes things much easier to focus on just one goal and maybe a subgoal. Make dinner time just about eating for those who are hungry. If he's hungry before dinner, let him have something to eat and then let him join you at the dinner table if he wants. Let him bring something to the table that interests him. If you want him to be with you, make it easy for him to be with you :-) Don't put obstacles in his way that would drive him away.
Coinciding family time with dinner time is convenient. Imposing it, though, is trying to create a Norman Rockwell illusion. If you want everyone together, then figure out ways that people will want to all be together, rather than creating an atmosphere of force that will make them want to flee as soon as possible.
Table etiquette isn't rocket science. It doesn't take 17 years to learn. ;-)
My daughter used to love to sprawl on the table to eat. In many parents' eyes they'd assume she wasn't learning good manners and she would assume sprawling on the table was "proper etiquette". And, yet, even though I never told her not to, she never did it at anyone else's house nor at a restaurant and did eventually out grow it. Not all kids are good at picking up social cues on their own like that. Some need our help. But *all* people are perfectly capable of understanding that different places call for different behavior. They can understand that appropriate behavior in one place can be inappropriate other places.