I'm sorry to hear that Buddy is so sick.
Of course, you know, that the best thing for him is veterinary care. Ideally, he would be on intravenous fluids, antibiotics, and something to stimulate his appetite. He needs a thorough physical exam and blood work in order to figure out what is wrong.
Without veterinary intervention, unfortunately, his prognosis is much worse. However, you'll have to give it your best effort and hope that he is able to hang on until you can get him in somewhere.
It is vital that he eats and that he doesn't become dehydrated.
If he is not interested in his regular food
, you can try offering tuna, or chicken, beef, or turkey baby food with no garlic added. If he is not willing to eat any of these things, then I recommend that you make a slurry of some canned food by mixing it with some water. Use a turkey baster, eye dropper, or baby oral syringe to slowly feed him the slurry. Put small amounts in his mouth, giving him time to swallow it, before adding more. Ideally, he should be eating at least 10 ml (2 teaspoons) at each feeding. Feed him every 1.5 hours.
He will get some water into him via the slurry. You can also give him some Pedialyte via syringe every 2 hours to help maintain his electrolyte levels.
If you are able to without getting bit, try to look into his mouth to determine the source of the drooling. A jaw fracture, tooth problem, laceration, or foreign body stuck in the mouth could cause this. Drooling can also be due to nausea or kidney
Make sure that he is able to maintain his body temperature. If he feels cold, use a hot water bottle (not a heating pad, as these can cause burns) to help keep him warm.
Hopefully, your little guy can hang on until your vet opens.