I saw him maybe four days ago playing with something on the farm road on the other side of the Eastern field of purple loosestrife that is now brown. Don't know what it was but it looked at the time like he was eating it. He hasn't been out since, and he started acting sick the next day.
The sun is now out (after hiding behind gray things) so I'm going to take an exploritory... He mostly wants to sleep, has not been eating, doesn't drink much (not good), and when he used the litter box last night the urine smelled really strange ...
Antifreeze is the first thing that comes to my mind. If you can, get him to the vet for charcoal before the kidneys shut down.
Very sorry about your kitty!