The vet shouldn't have left you with this problem, and this worry. OK so they didn't want to remove the gallbladder. Fair enough. But they should come up with an answer for treating this, or tell you more info.about what could be the cause. Get back to them and ask. They shouldn't have left your dog in pain, and you with no answers!
Or you could take your dog to a different vet.
That sounds like something my past vet would have done! I recently switched my German Shepherd and my mother's Miniature Schnauzer to a different vet and was quite satisfied. Maybe you need to get another opinion. Either way, if your dog is in pain, they should run tests and ask questions until they find the cause. One thing that concerns me... your Schnauzer is only 6 pounds?!?!!? Breeders of "toy" "teacup" and "micro" Miniature Schnauzers have a lot of problems in their bloodlines because they inbreed the smaller ones so much to get that size. The result is an off-looking Schnauzer and often frail and/or sickly little dog. Toy sizes are also prone to typical toy problems like hypoglycemia and of course frailness. One of the things that Minis can be plagued with is pancreatitis. I would think they would have found that with exploratory surgery though. Maybe he got hurt? Tiny dogs get hurt very easily and it could literally be anything. As far as his gallbladder goes, sometimes liver and/or heart problems are also present with an enlarged gallbladder, so you might want a liver enzyme test and a cardio test. That's something that wouldn't be found with exploratory surgery. Tell your vet that you're disappointed and you'll look elsewhere for help if he doesn't help you. Remind him of what his job is for and if he doesn't do it, go somewhere else. I've found too many vets myself that make me wonder why they became vets at all, but there are good ones out there. Good luck to you and your little buddy. I hope you can get a vet to find the cause so the little guy can stop hurting! Until then, you can give a dog aspirin, BUT you should get his liver checked first because of the gallbladder, AND you should ask a vet how much to give (very important)! Just try to keep him comfortable (as I'm sure you are) with blankets and a lot of love until you find out what's bothering him.