Thanks so much for the additional information and clarification:
First of all, I hate to say this, but the anesthesia and anal gland expression, but especially the use of an antibiotic injection was a bit over the top for a cat that was showing absolutely no symptoms of anal glad impaction (scooting, licking, etc).
Even if he did actually have impacted anal glands, this usually won't cause diarrhea. In fact, it is the exact opposite. It is the pressure of normal bowel movements that help release anal glands. So if a pet has diarrhea to where there isn't much pressure when voiding, then the glands may become impacted. Now, dry food and obesity can predispose to impaction, but I have some suggestions for that too.
The combination of stress, anesthesia, vaccine, antibiotic injection (especially without infection because impaction does not always mean infection), then more antibiotic, could cause diarrhea in any pet.
To determine if antibiotics were warranted, I would ask your vet if the glands were full of normal contents, or if they were obviously infected (indicated by watery or yellow/greenish discharge or blood).
Assuming the diarrhea was caused by all of the above, I would continue the Fortiflora as this is a good way to repopulate the intestinal tract. If the glands were infected, metronidazole is a very limited spectrum antibiotic and I have had better luck treating actual infections with something more broad spectrum such as marbofloxacin or enrofloxacin.
The mucus in his stool does indicate inflammation of the colon, likely the cause of the diarrhea. This is one thing that metronidazole does help, although I don't like using antibiotics unless really needed.
I would absolutely talk to your vet about taking him off the steroid. They are potent antiinflammatories, but are also probably suppressing his normal immune system response and may actual inhibit this from resolving. You may need to wean him off this drug by reducing the dose to every other day so ask your vet first.
The unfortunate thing about dry food is that many medical conditions are thought to be linked to not only the lack of moisture in dry food, but all the grains (corn, wheat, soy, rice) which are used as fillers to increase the profit margins of the food companies. Cats are very strict carnivores, and plant protein is a very low quality protein source and can also be very irritating. Please visit www.catinfo.org to learn more about this. Most feline and internal medicine specialists are now recommending feeding canned food and grain free brands such as:Innova Evo, Wellness Core, Pinnacle, Before Grain, and Weruva. These are available at private pet stores.
Not only does animal based protein make more sense for cats, it can help with prolonged diarrhea and really help with weight loss (think Atkins diet). The sheer moisture in canned food really helps them feel full. The reason cats act so hungry on dry food that is also high in grains is because grains= carbs. If you eat a lot of carbs, your blood sugar skyrockets, your insulin levels rise, and an hour later you are hungry again.
I hope this gives you some things to consider. Please let me know if I can do anything else. If you accept this answer, you will only need to pay once, but I can continue helping you.