I am sorry to hear about Fin.
My name is Rebecca. I am a veterinarian, and will do my best to help.
The first thing I would do, if he were mine, is a microscopic examination of some of his stool under a microscope.
Most parasites are too small to see with the naked eye. If he has something like Giardia, he could spread that to family members, and you would not see anything in the stool except diarrhea, mucus and maybe blood.
The All in One dewormer would not help with Giardia.
If he has something like Giardia or Coccidia, he needs prescription medication.
He could also have something else, such as low pancreas function, a food allergy to something he is eating, or an infection.
I am not seeing responses from you. I hope you can see what I am writing!
I will try to connect with you another way, if I do not hear from you.
If he is drinking more water, that is probably to make up for losing water in the soft stool.
Thank you for your response.