I've had a cat with a blockage, and it doesn't sound like that. I've also had a cat with FIP, and they almost always have a high fever. Can you do an elimination diet and try and see if there's something he's sensitive to? (I am assuming you don't give him cow's milk...that can cause those symptoms too.)
Also, canned pumpkin (unspiced) is good for both diarrhea and constipation. What food is he on now?
Not sure what he is on at his vets but but prior to all this he ate Fancy Feast wet usally mixed with very litte Pro Plan dry for kittens.Now and then he had 9 lives dry essentials. He does not drink milk and thru this ordeal he never did have a fever. Also for a previous post we have no house plants.
I'm not sure about how to go about an elimination diet, but I would tend that way. For example, boil chicken, and he gets only that and see how he does on that, etc.
He was sent home yesterday, no special food however with his evening meal I am to add a powder that encourages his intestines to work better. He stayed at my moms yesterday since my wife and i both work my mother picked him up. I brought him home this morning. Seems to be doing great, eating playing and going to the llitter box and getting caught up on his rest. The poor thing was thru so much this week. Thanks for your prayers and that God for his part in this too.