I'm sorry to hear your budgie is sick and from your description it really seems he is in need of an emergency appointment at the vet's.
Till you get him professional help, you can keep him warm by placing a warm fleece blanket over the cage. You can also place a warm water bottle under the cage and covering half the bottom of the cage, so that your budgie can move away from the heat if needed.
You can offer him spray millet soaked in electrolyte (Pedialyte) to try and give him a little boost.
If he is tame, you can gently clean up his vent area from the poop build up with a cotton bud soaked in lukewarm water.
Where do you live, if you don't mind my asking? Have you tried looking online for avian or exotic vets around your area of residence?
Given your budgie's current state, this is an emergency situation and you need to take him to a vet, even if the emergency vet is out of town.
I'm wishing you the very best of luck with your budgie and I hope his condition will improve soon and he gets back his health.
RIP sweet Tito (Summer 2008 - January 17th 2013).
You are missed and never will be forgotten.