Thank you again,
First, I am glad to hear that Panacur was used and do hope that they did at least get a proper dose. The reason why we'd want to use that wormer specifically is because it is one that dose help with worms but also some of the potential protozoal causes for diarrhea in cats.
Now it does sound like you are on the right track with ruling out potential triggers and narrowing down causes for his signs. And if the antibiotic was only started today, then we will need to give this time to take effect. Still I do appreciate that this long term continued diarrhea will be taking a toll on him (from a view of hydration and nutrient absorption for his growth) and yourself.
Now while Prokolin tends to be quite a good product for soothing GI microflora (good bacteria) populations and slowing diarrhea, I do find that for severe cases the amount of kaolin it contains may not be enough to slow the loose stools. Therefore, I would note that you could speak to your vet about using a plain Kaolin/Kaogel product (More Info/Dose). They can dispense this for you but it is also available OTC at most chemists. So, if he is still very runny, you could consider trying this with him. And I would just note, don't be tempted to use any others like Immodium or Pepto Bismol, as these can be harmful for cats.
Otherwise, you mentioned quite a varied diet for your lad. And while this is fine when stools are normal and all is well, it is less then ideal for situations of diarrhea (since quite diet changes can worsen diarrhea too). Therefore, I would advise sticking to one diet. As well, since we have compromised absorption, I would note that you could consider putting him on Hill's I/D (LINK), Royal Canin Gastrointestinal (LINK) or Royal Canin Sensitivity control (LINK). These can all be purchased from your vets or online. All are designed for easy absorption in situations like this (and I am sure you can appreciate, the more he absorbs means the less that can be passed as diarrhea).
Finally, since diarrhea can cause dehydration in kitties (and they often cannot keep up with fluid losses) I would advise considering checking his hydration. To check his hydration status to make sure he is not becoming dehydrated there are a few things we can test. One is whether the eyes appear sunken, if the gums are tacky instead of wet/moist, and whether he has a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a wee video on this HERE. (They use a big dog but the techniques are the same). If you are seeing any signs of dehydration already, then we may need to think about fluid therapy from his vet (since it is often the dehydration that starts to tap their energy level and depresses them).
If you are concerned that he is becoming dehydrated despite drinking, you can try and encourage him to do so more by offering fresh water or even low-sodium chicken broth. As well, the reason I noted wet food diets is because these will have more fluid content then dry and help match his losses. Otherwise, if he isn’t amenable to these, you can syringe feed pedialyte or Lectaid (from the vets). These are nice options to give because they will get some of those lost electrolytes back into him as well as fluid. A typical maintenance rate for hydration in an animal is 48mls per kilogram of weight a day. If you do give syringe fluids, this should obviously be divided up into multiple offerings through the day rather then all at once. This value will give you the total he needs for the day and is a good starting point to give you an idea of feline daily requirement. (we aren’t calculating losses, so you can add an equivalent volume to match how much diarrhea he is producing). If he vomit when you have given this, then therapy should be discontinued (since we don’t want vomiting because of our intervention).
Overall, if the Verflox was started today then hopefully we will see this start to settle. Otherwise, we want to ensure we are feeding an easily digestible diet to reduce the volume of what can be passed as diarrhea and monitor hydration. Further to this, since the Prokolin wasn't strong enough for him, do consider swapping to Kaogel/Kaolin to better bind the loose feces, slow the diarrhea, and give you a chance to reduce this as you treat the underlying cause.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,