Hi, E.. Well, it sounds like he's doing this for attention. Has it been just since the birth of his new baby brother? When a child feels his position in the family is taken away or threatened, he will often go to really bizarre lengths to recover his sense of importance. This leads to a huge amount of negative attention. At least he hasn't reverted to lots of infantile behaviors...but it sounds like he's noticed that an awful lot of attention is paid to his baby brother at feeding time. It sounds like he's trying to steal back some of your feeding-time attention.
I think that the remedy for his strange behavior might be to place less emphasis and stress on mealtimes. In other words, give him lots of attention at times besides mealtime. If that seems to be the only time the child's parents spend with their child, then all the child's attention needs are filled at that time -- and eating times become critical because that's the only time parents are available to give him attention.
So #1, give him attention at times when no food is involved, unless of course it's a family outting and he's getting some ice cream or popcorn or whatever. But give him attention for doing other things besides eating.
#2 -- if he were truly hungry, he would not be holding the food in his mouth. You may be trying to feed him when he's not hungry. I understand that kids need structure, and they need to get used to regular mealtimes, but trying to feed him more than he actually needs will cause not only behavior problems, but physical problems as well.
#3 -- give him smaller portions. You can always give him seconds if he's still hungry, but let him eat a little bit of this and a little bit of that, and when he starts acting badly at the table, take the food away. If necessary, let him sit there and watch everybody else eat and socialize with the family, but if he's actually full, then trying to make him eat a few extra bites is unnecessary. He may be trying to tell you that he's full, and eating more makes him uncomfortable.
#4 -- if you're worried that he will need a snack in between meals because he's not cramming enough in at each meal, please remember that a child his age NEEDS SNACKS ANYWAY. No matter how much he eats at regular mealtimes, his metabolism and his activity level (if he's a healthy and active 2.5 year old) makes it absolutely necessary for him to snack here and there. So let him eat the small meals that satisfy him at regular mealtimes, before he starts doing the grotesque holding behavior... and then a few hours later, he can have, say, 1/2 an apple or 1/2 a peanut butter sandwich. Don't let him get away with leaving the table and then 1/2 hour later, "Mom, I'm hungry," but he wouldn't eat with the family.
I'm not suggesting that you allow him to demand food at all hours of the day and night, but you have to understand the size of his stomach and the way it fills, and the speed at which he uses up this food. He may just be a human being who needs to eat 4 or 5 small meals a day rather than 3 rigidly-set meals. An adult can cope with 3 rigid mealtimes, but very few children can do this.
I hope this is helpful, and that your son and you can resolve your power struggles at the table. Basically, these kinds of power issues are resolved by taking away his reward for doing the bad behavior and offering a positive alternative instead (like good attention away from the food issues).
Peace,
Syl