You've got an extremely sick Budgie, by the time they are at this stage where they are lethargic, fluffed, and at the bottom of the cage, they have been sick for quite a while, and are at the end-stages of such..without the help of a Certified Avian Vet immediately, he's not got long left. I don't mean to scare you or be forward...well, yes I do because it's that serious....Budgies are so good at hiding illness for such a long time that by the time we see outward signs of them being sick, it's often too late. So if you haven't yet gotten him to a CAV, you need to do it right now, or to an emergency Exotic's hospital right now, whichever, as he's not got long...
Keep him warm (they are from Australia, they don't mind heat and he needs to be kept very warm while he's ill) and offer him lots of water and food if he'll eat it, but I doubt it at this point...If you know how and you have an oral syringe or any type of plastic syringe without the needle on it, obviously, you can dissolve some sugar in water and try to get him to accept that from the syringe, but do not try to force it into his mouth, you could end up aspirating him. Just try to drip it on his beak for him to lick off, he needs the fluids and the sugar right now...