Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
Poor wee Basil!
The normalizing of his stool color is a positive for Basil and may be due to the anti-parasitic treatment you have been using but it won't be the cause of his diarrhea. So, if his stools are still very loose at this stage, we do have to be concerned that either the trigger for Basil's diarrhea has not totally been cleared by this treatment or that there may be more then one thing amiss here.
In this situation, I would suggest that you can try to slow his diarrhea using a cat safe anti-diarrhea treatment like Kaolin, or Protexin Pro-kolin. Both can be dispensed by your vet, but the Protexin product may be a better option here. It is available over the counter at your vet's and contains not just an anti-diarrheal treatment but also pre/probiotics to restore GI bacterial imbalance (which is likely playing at least a partial role in his diarrhea). So both would be options but the Protexin may be a better choice under the circumstances for Basil.
Further to all of this, if you have not already, you may want to consider putting him on a light/easily digestible diet. Examples of this would be boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), meat baby food (do avoid the ones with garlic powder in the ingredients) or there are also veterinary prescription diets that can be used in cases of gastroenteritis (ie Hill’s I/D or Royal Canin’s sensitivity). These diets aim for provide nutrition to the stressed gut without making it work hard to digest the food. I would advise frequent small meals to minimize strain on the already stressed gut, and to help lower the volume of diarrhea.
As well, even though you report that he is drinking well, do keep a close eye on his water intake and his hydration. Cats cannot keep up with fluid loss via diarrhea for the long term and dehydration will be what starts to make him feel poorly. To check his hydration status to make sure your cat is not becoming dehydrated there are a few things we can test at home. One is whether the eyes appear sunken, if the gums are tacky instead of wet/moist, and whether the pet 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 it makes it easier to see and the principles are exactly the same) If you are seeing any signs of dehydration already, then you do want to have your kitty seen by his vet before this gets out of control for him.
If you are concerned that he is becoming dehydrated from the fluid loss via diarrhea, you can try and encourage him to drink but offering fresh water or even low-sodium chicken broth. If he isn’t amenable to these, you can syringe feed pedialyte. Pedialyte is good here (though aim for a flavourless one since cats don’t love fruit) because it will get both fluids and lost electrolytes back into your kitty. A typical maintenance rate for hydration in an animal is 48mls per kilogram of weight a day. If you do give syringe pedialyte, 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 the feline daily requirement. (we aren’t calculating losses, so you can add an equivalent volume to match how much diarrhea is being producing). If he vomits when you have given pedialyte, then therapy should be discontinued (since we don’t want vomiting because of our intervention).
Finally, if his diarrhea has not settled with the Panacur alone, we do have to be concerned about other agents (bacterial like E.coli, Salmonella, Clostridia, Campylobacter; viral agents like rotavirus, panleukopenia; or protozoa like Giardia, Tritrichomonas, Coccisia, etc). Therefore, if he is not settling do consider submitting a fecal sample to your vet for testing. This can be sent to the lab to rule out those common infectious agents. Depending on the test findings, you will be able to identify which is present and therefore take steps to target it effectively and get Basil's stools back to normal.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,