Cat people, I need some advice, sick kitty

My 15 year old cat had bloody mucous phlegm and also mostly blood feces the last two days.
I took him to the Vet the first day and she ran some tests to try to determine what was causing it.
She did a full blood panel to see if there was anything going on that would show there. I also wanted her to take an x-ray which she did and nothing showed up.
She did a stool sample and no parasites were found.
Chauncey is mostly an indoor cat who goes outdoors onto the porch and sits on a cushioned chair sometimes. He doesn't go into the yard.
I was worried he may have got something contagious after that stray was found climbing the fence.
He didn't have a temperature when I took him in either. This has me stumped.
She gave him some sub-Q fluids and some medication for bacterial infections. He has to take the meds for five days and then to call her if he is not better.
She couldn't believe his blood work was so good for his age, she said it looked like that of a two year old.
I had just commented to my husband a couple days before this, that Chauncey has really never been sick at all since we got him, 14 1/2 years ago. And then this.
He has been in one of the bathrooms with his own litterbox so I can monitor him. He seems to be getting better, but still won't eat. She also gave him some prescription ID canned food which she said was highly digestible and easy on the stomach.
I wish there was something more I could do for him and thought maybe there is someone here who can offer some advice they may have personal experience with.
He can't get to any poisons as I don't use them, or harsh chemicals to clean with.
The only thing I can think is maybe he chomped on a bug or a frog or lizard which sometimes get on the porch. But whenever I have checked on him, he is always just sleeping in the chair there.
This was not his regular Vet, but she is the one who helped me to pull Petey through his Parvo last year and I trust her to know her business.
One of the other cats threw up yesterday and there was a little earwig in his vomit. Do they irritate the stomach? That cat is fine, though aside from throwing up. The earwigs come up from the crawl space I think as they are always finding them in the bathroom.