Grinding his teeth is a sign of pain; the pain is most likely why he's not eating. It's a medical emergency if a bunny goes 8-12+ hours without eating, drinking, pooping and/or peeing... however, even if it hasn't been longer than that, I would skip the home remedies and get him to the vet immediately if at all possible since he's clearly in pain.
If an immediate vet visit just isn't possible for whatever reason AND he seems to be pooping at least some (which means he doesn't have a total blockage), then you can try some at-home remedies to tide him over until you can get him in:
You can buy a syringe at any grocery store. If you have critical care food (comes from vets), you can syringe feed that. If you don't, you can make a pellet slurry - take some pellets and add water or pedialyte; microwave. Let sit for 5-10 mins so the pellets can "fluff", then mix together (breaking up the pellets as much as possible); add more liquid if needed to get it to a good pudding-like consistency for syringe feeding (using a food processor (before adding liquid to the pellets), a blender or a mortar and pestle (again, before adding liquid) will make it easier to break up the pellets). Critical care will work with a syringe as-is but with the DIY slurry, you may find it necessary to cut the tip of the syringe off to make the opening larger.
You can also offer a little bit of canned pumpkin (PLAIN stuff, not the pie filling kind) and/or some fruit or squash flavored baby food... not a whole lot, as these foods are sugary - maybe 2 tsp or so. If he won't eat the pumpkin on his own, you'll need to water it down with water or pedialyte in order to easily syringe feed it. These things could also be mixed into the pellet slurry or critical care to make it tastier.
You should syringe water or pedialite if you're worried he might not be drinking enough and/or if you're feeding pellet slurry/critical care.
When you syringe feed a rabbit food or liquids, you want to aim for the gap between his front incisors and back molars and always squirt the food/liquid in sideways - never straight back - to make sure he won't accidentally inhale it. Remember to give him time to chew/swallow. I find it easier (especially with liquids) to have the bunny wrapped in a towel, as things can get pretty messy.
Here's a great video showing how to syringe feed - [ame]http://www.youtube.com/watch?v=8iGZVYVm5Bg[/ame]
You can also offer pain meds to make your bunny more comfortable - metacam/meloxicam if you have it (bunny painkillers); baby aspirin or low-dose aspirin if you don't. [Don't forget to mention the amount(s) of pain medication and time(s) given to the vet if you end up going.] This website has dosing info for pain meds -
http://www.medirabbit.com/Safe_medication/Analgesics/safe_analgesics.htm
Simethicone (any brand of over the counter baby gas medicine) can also be given. it may or may not help depending on the cause of the GI stasis, but it's *incredibly* safe for rabbits so it definitely can't hurt. You can give 1-2 cc every hour for 3h, then 1 cc every 3-8h as needed if it seems to be helping.
Probiotics are also very helpful for tummy troubles (stuff like benebac/probios) - you can sometimes find them at pet stores or feed stores; failing that you can buy them online. Do NOT give yogurt containing probiotics, as rabbits are lactose intolerant, plus the extra sugars can make stasis worse.
