Cat spooked of a balloon, he is traumatized and I’m at a loss

First of all, if your cat is still not drinking, eating, or going to the litterbox, take it back to the vet. Not doing these things for so long is very serious.

Unless your cat is normally very skittish, nervous, or easily spooked, I don't think the problem is that it's scared of the balloon. For one thing, some cats might be scared of balloons, but it's definitely not like, say, the vacuum, which is just about universally terrifying for cats. But even with the vacuum, the normal reaction is to hide as long as the vacuum is out, and then calm down quickly once the vacuum is put away. A very nervous cat, like a feral cat, might hide for longer, but you describe your cat as being social. Socialized cats don't typically hide like that unless the thing that's bothering them is still a problem.

Such a radical change of behavior, sudden aggression, not taking care of its normal bodily functions, sounds like a medical problem to me. At the least, I would try to think on if there have been any other changes, or any other unusual things you've observed with your cat that might tell you what the problem is.

i dont really have an answer for you, but more of a shared experience. My cat is absolutely terrified of balloons. He likes to stay in the bathroom most of the time, but my daughter just had her second birthday and I blew up many balloons and made a balloon tower. he wasn't scared until he popped a balloon. after that, he literally ran for his life. I've never seen such terror. and my boyfriend didn't help, he thought it was funny that the cat was so afraid, so he started tormenting him with the balloon tower. my cat confined himself to the bathroom and wouldn't move or eat or anything. its been a week now and he's mostly back to normal, but the terror that I saw in him made me so sad. He scratched the crap out of me when I tried to bring him to the living room. i don't know what it is, but I saw something on Quora saying that they might be able to see minute vibrations in the balloons with no apparent cause and that might be what scares them.