A few questions -

Have you seen any of them straining to go to the bathroom? Have there been any other symptoms you've seen - losing weight, change in appetite, drinking more, more lethargic, peeing more frequently?

Have you gotten blood tests done? I'd definitely ask to do a senior blood panel on the 5 yr old, as well as a fecal exam. I think he's probably the one having the issues. The peeing outside of the box, loose stools, diarrhea and the blood says to me that there's something going on with him to me.

Why did they put him on antibiotics? Did he test positive for something?

There's so many things that could cause these symptoms.

I hope it's nothing major and he gets better soon. We all know what it's like to be worried about your kitties. Hugs to you.