ooh i am very big on manners at the dinner table lol so I would be being super strict with this.
take him to the toilet before dinner.
make sure everything he needs is at the table (really just plate of food, drink and cutlery)
then sit down and eat. if he gets down tell him that you will count to 3 and if he is not sitting up and eating then his dinner time is over.
I would absoltuely NOT stand for tantrums, trying to steal food, hitting, screaming etc.
I mean, dealing with the tantrums is probably a whole other thread, so won't go into it here, but we usually take ds1 upstairs to calm down if he has got himself that worked up.
then I would give him one more chance to eat his dinner nicely before removing it completely.
I do tend to keep ds1's and if he says he is hungry later I offer to re-heat it for him.
you need to take care of yourself and make sure you sit and eat your meal with ds2, even if ds1 is playing up.
let him know thast he won't get away with behaving like it.
just one last point... do you think he is feeling left out in any way what with a younger sibling and a new one on the way? perhaps this is his sure-fire way of getting your attention all to himself?
perhaps if ds2 has a nap in the afternoons you could spend some real quality time with ds1 doing whatever he chooses to do?