Hi there and welcome to Just answer!! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
The problem is that you do not know why JT has this diarrhea. The treatment must be aimed at the cause. Because of his age, there is a possibility that he could be suffering from an intestinal lymphoma and for this, there is no treatment.
Since he has been wormed recently, there is a chance that he could be suffering from Trichomanas which causes this severe diarrhea due to his system being immune suppressed, Your vet can diagnose this by evaluating the stool so it is best to have your vet examine this. Treatment is metronidazole and will be effective.
there is also a possibility that JT's digestive tract has been irritated by the worming and this will resolve with anti diarrheal agents that your vet can prescribe.
have your vet examine JT to be sure this is not serious.