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 -
http://www.youtube.com/watch?v=8iGZVYVm5Bg
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. 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 w/probiotics, as rabbits are lactose intolerant, plus the extra sugars can make stasis worse.
Mind you, the above at-home remedies are NOT a replacement for taking the bunny to a rabbit-savvy vet to figure out the underlying cause of the GI stasis! If your bunny regains (and maintains) his normal appetite within half a day or so, you can skip the vet trip but if you continue having to force-feed him, a vet is a must.
here's a good listing of rabbit-savvy vets -
http://rabbitsonline.net/view_forum.php?id=9