Sitting hunched and grinding teeth is a bad sign. The antibiotic may have severely upset his stomach. Usually when this happens they also refuse to eat, which is also bad if it goes on for too long. I can only say what I might do in a similar situation when I didn't have a good rabbit vet that I could take my rabbit to for treatment. You'll have to decide if you think it might also be the right thing to do for your rabbit.
Certain antibiotics can sometimes cause bad bacteria to grow in the rabbits digestive tract and this can make them very sick. When this happens, the medication that I would want to give my rabbit to stop this overgrowth of harmful bacteria, is called metronidazole at a dose of 20mg/kg every 12 hours for 1-3 weeks. This is what I would want to give, especially if there is any bloating, stomach gurgling, and/or diarrhea(pudding or watery consistency).
Since the rabbit also stops eating when this happens, I would also start syringe feeding, but first I would check to make sure my rabbit wasn't bloated(as you shouldn't syringe feed a rabbit with bloat). To check for bloat, I
very carefully(not using too much pressure) feel the rabbits belly, just under the rib cage, to feel if the stomach is bloated. If it is then it will feel like a small inflated balloon just under the ribs. If the stomach is bloated, I would give 1ml/cc of simethicone infant gas drops(with rabbit safe ingredients)for 3 doses an hour apart, along with the medication metronidazole that I was already giving to control the bad bacteria. If there is no sign of bloat then I would go ahead and syringe feed.
For syringe feeding I use oxbow critical care food mix for rabbits, but if this isn't available a pellet mush can be used by soaking the rabbits pellets in warm water making it into a pudding like consistency or a bit thinner. I would then use a large tip feeding syringe and carefully squirt it in through the corner of the rabbits mouth, squirting a small amount at a time, making sure the rabbit has time to chew and swallow, as it's important to be very careful not to squirt too much too fast or the rabbit might breath it into it's lungs. I would also syringe some water as well.
If I couldn't get the medication metronidazole, then I would just give the simethicone, if I found a bloated stomach. If the stomach wasn't bloated then I would just syringe feed food and water if my rabbit wasn't eating, until my rabbit started eating again on his own.
So this is what I think may have happened to your rabbit, it's called clostridial enterotoxemia. It's an overgrowth of the clostridium sp. bacteria. It can be fatal and needs immediate treatment when it happens, to have any chance of saving the rabbit affected. This link has the information about the disease and how to treat it with the antibiotic metronidazole, and the dosing information. It is somewhat technical though, with some medical wording. If you do have access to metronidazole and decide to give it, if it is oral medication, then you need to make sure that the suspension liquid that it is mixed in with, is safe for rabbits. It for sure shouldn't have any xylitol in it.
http://wildpro.twycrosszoo.org/S/00dis/Bacterial/Clostridial_enteritis_rabbits.htm
http://www.medirabbit.com/EN/GI_diseases/Generalities/Enteritis_en.htm