Hi, P.. I have not had this problem specifically with my son, but the only time I ever had problems with him throwing up at the table were when we had conflicts over food (trying to force him to eat things and certain amounts of food when he wasn't willing) or when the family had fights at the table over other things.
I suspect if this is a behavior that he did before and he stopped, that the reason he's doing it again is for attention. If he's not getting enough positive attention at other times of the day, then he certainly can turn on the bad behavior and get everyone to notice him at the table.
My mother, God rest her soul, claimed that one of my older sisters would purposely vomit at the table to cause a huge bad scene when she was angry at my mother.
I agree that you're going to have to go back to the earlier discipline of giving him very small amounts of food in small pieces. In addition to this, I think you need to make sure that he gets plenty of attention at times when he's totally away from the table. Make sure that he gets loving attention and play time with parents and with the older brother.
In the meantime, I would also spend some quiet time with him to try to draw out of him what might be bothering him. Something might have happened at school that triggered this behavior again, or he might even have some conflict with his older brother. We always seem to assume that the youngest child gets all the attention, but I myself was the baby of the family, and I was forgotten about all the time.
BTW, make sure your little one doesn't watch any disgusting movies where people vomit for laughs. Animal House is one of those movies; some of the videos in the Jackass genre are similar. Even cartoons can show people vomiting while the kids are supposed to laugh at them. It makes no sense to me, but it seems that everything that comes out of the body is funny to young kids.
I hope everything works out for the best and you always have peace at your table.
Blessings,
Syl