Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
I do apologize that your question was not answered before. Different experts come online at various times; I just came online and wanted to touch base with you.
Given his diarrhea is sounding severe, his being sluggish is understandable. Now our main suspects here for Cloister's diarrhea would be a gut infection (eg viral, bacterial, protozoal, worms) or dietary indiscretion. So with that in mind, we can try some home supportive care just now. To do so, let's put him on an easily digestible diet like boiled chicken, boiled white fish, or meat baby food (as long as its garlic/onion free). The aim of these diets is that they will be better tolerated/absorbed by the compromised gut. Therefore, it should get more nutrients in and result in less GI upset/diarrhea. Fiber (ie canned pumpkin) and OTC probiotics (ie Benebac, Fortiflora) can be added to these meals to firm those loose stools quicker and support digestion. And we can plan to slowly wean back to his usual diet once he is settled.
As well, since he sounds to be very loose, you can try a pet safe anti-diarrheal too. As I am sure you appreciate, these would not be a cure for infectious issues; but it can still be of benefit. It will reduce diarrhea load, allow the body to absorb more water/nutrients, and soothe the upset gut. In regards ***** ***** options, the one we most commonly use is Kaolin/Kaopectate (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/kaolin-and-pectin-kapectolin-k-p)). This is available OTC at most pharmacies (no imodium or pepto for kitties). Otherwise, Propectalin, Canikur, Fast Balance, and Protexin Pro-Fiber (all OTC at vets, pet stores, and even Amazon) would be another option. All will slow diarrhea and those last ones have the added bonus of providing support to the delicate good GI bacteria. So, these can be used as a short-term means of soothing his upset GI. And also if he hasn't been wormed in the past month, that too would be a good base to cover. OTC Fenbendazole/Panacur would be a good option as its good against worms and can tackle some of the protozoal agents too.
Overall, these signs do raise some concerns here. Therefore, we’d want to start supportive care to settle Cloister's stomach. If he doesn’t respond to the above within 24 hours; then we'd want to get a local vet involved. They can assess his hydration, test a stool sample, make sure there is nothing in his stomach that shouldn't be there or any sinister viruses present. Depending on their findings, his vet can treat him with antibiotics +/- anti-protozoals as needed to nip this in the bud.