I know how hard this is for you. You're doing ALL the right things though. He's got a very much higher chance of getting through this with the care you're making sure he's getting.
Bacterial infections include: Chlamydia, mycobacteria, salmonella, pasteurella, yersinia and others. Tests should include sampling (and culture) of the nares (nostrils), oral cavity, cloaca, etc., but if the vet isn't knowledgeable about the normal flora in the bird species they are examining, the results may not mean much. Inexperienced vets will find that the culture grows something and often base their diagnosis on what they know of mammals. Cockatiels are known to contract Bacterial Enteritis - caused by salmonella, E.coli, coliforms and other bacteria infecting the intestinal tract. Common symptoms include diarrhea and going off their food; fluffing, lethargy, picking at and scratching their vent (bottom end), preferring the bottom of the cage and perhaps what looks like seizures (shaking, shivering).
Not all birds will show symptoms. Some are carriers.
The good news is that even a very sick bird can recover from this with a quick veterinary intervention and administration of the proper antibiotic (which will depend on the type of bacteria is found).
Exposure to other sources of contamination like the infected feces of another bird may also occur.
Of course he looks sad. He's stressed and he's scared, but when he comes back feeling much better it'll be worth it.
Just be prepared for him to act a bit 'off' for a few days, even up to a week. These little birds with great big hearts and bigger personalities may take a while to shake off the insult of a hospitalization like this. Tender loving care, calm surroundings and all the indulgences you want to give him will help.
Let me know what they find and I'll be pleased to go over it with you as much as you need ok?
And I'd say 'don't worry', but I know you're going to anyway. At least don't worry as much. He's in good hands!