A simple inexpensive test for yeast ( around 30 bucks ) anyyyyyyy vet can do even if they aren't experienced glider vets is a tape test. The vet takes a piece of scotch tape presses it against the gliders belly, puts a drop of dye on the tape and looks at it under a microscope. Yeast looks like little peanut shapes under the microscope and is very easy to find if there is a yeast infection.
If it's a yeast infection the glider will probably stop grooming themselves, the fur feels greasy and looks gross. The treatment can be a long 6 week course of meds and can take 3 weeks to even start seeing improvement.
With a different dye they can also see if there is high levels of bacteria with the tape test, but they can't tell what kind of bacteria it is or what medicine the bacteria is resistant to. For that you need the vet to do a bacterial c & s with sensitivity test.. The sample is usually sent to a lab, the lab grows the bacteria then figures out which medication is going to work the best. This test is a bit more expensive mine was around 150.00 and can take a couple weeks to get results from.
Bacterial infections on the belly can also be cause by oral issues, if there is a oral infection the glider spreads the bacteria to other parts of their body when they are grooming. This may not be this gliders issue, but is something else to look at and rule out.
What kind of water are you giving them tap or bottled?
Since peeing is causing him pain I would suggest a urinalysis being done. Uti's are kind of pain to get rid of because they are so easily reinfected. You have to break the link that is reinfecting them usually by cleaning the cage and whats in it daily while the glider is being treated.
The diet, that's a lot of protein gliders don't need 50% protein they are also getting protein from the bee pollen. Bee pollen if being fed whole can give false positives for parasites when fecal tests are done so please inform any vet that does fecals on your gliders bee pollen is part of their diet if you continue to feed the way you are.
Calcium..... how much are you giving them? to much can be just as bad as not enough.
I would suggest researching some widely used diets that are more balanced than free feeding. The 2 I recommend are Bml and Critter love. Bml is by far easier of the 2 to make. Diets like these provide calcium, vitamins and minerals, protein etc all in one evenly distributed recipe, fruits n veg are fed with it.
http://www.bmldiet.com/bml-recipe.html
https://www.critterlove.com/