Oh no! Poor guy. Any idea what he might have gotten into? Had he been outside?
No idea, but I suspect someone's been poisoning rodents. (But yes, he'd been outside.)
Any chance it was thread or ribbon?
Not really, no. His platelet count was very low, and he wasn't coagulating. Combined with the puking and fever and stuff, some sort of poison seems pretty likely.
Finding an appropriately nutritive substitute food he'll eat is a bit of a challenge, however, especially with the travel and other restrictions in place. Most of them are just way too high in carbs (and what he was eating before was one of the lowest-carb canned food on the market). Hopefully that's just temporary, and he'll be back to the usual stuff soon, poor love.
In the meantime, he desperately resents not going out.