is he getting hay? hay is really important for their digestive systems.
if he's not eating, he's got GI stasis which could be caused by any number of things... a blockage, a hairball, gas, etc.
he's probably going to need a vet trip... how long has it been since he ate?
between now and the vet trip, the important things to do are to make him comfortable and to get some food in him. also, is he drinking water still?
metacam if you have it (pain killer from vets)/baby aspirin if you don't will help with any pain he's experiencing.
dosing info:
http://www.medirabbit.com/Safe_medication/Analgesics/safe_analgesics.htm
for feeding him, you'll need a syringe...
I'm guessing you don't have any critical care food (also comes from the vet), but you can make your own pellet slurry. mix pellets with some water (or pedialyte)... if you have a mortar and pestle, you can grind them up a bit first to make it easier to mix later. microwave for a couple minutes and then let sit for 5-10 mins so the pellets can "fluff". mix well, adding more water if needed to get it to a pudding-like consistency, then syringe feed. you may need to cut the tip off the syringe and use a pen or something to widen the opening a little if the pellet slurry is a little chunky.
you can also try feeding canned pumpkin (plain, not the pie filling kind) or watering it down a little and syringing it, or syringing fruit or squash flavored baby food. either of these could also be mixed in with the pellet slurry to make it tastier.
(if he's not drinking much and/or if you're giving a fair amount of pellet slurry, you'll want to syringe some water as well. I *highly* recommend wrapping him in a towel for this because it gets messy.)
when syringe-feeding, you want to aim for the gap between the front and back teeth and always squirt the food/liquid in *sideways* - never straight back, as you don't want him to accidentally inhale some.
great video on syringe feeding: [ame]http://www.youtube.com/watch?v=8iGZVYVm5Bg[/ame]
another thing you can try is simethicone (over the counter infant gas drops) - 1-2cc per hour for 3 hours, then 1cc every 3-8h as needed. depending on the cause of the stasis, the simethicone may or may not help, but it's incredibly safe for bunnies so at the very least, it won't hurt.
