It takes a while for rat poison and anti-freeze. He could have had a weak heart (stroke, heart attack, heat, etc) or had a brain problem (seizure, tumor, etc). Or something else could have been wrong with an organ. Internal bleeding is a possibility if he fell, got hit with something, or anything else that could cause trauma.
There's multiple things it could have been and the only way to know is for a vet to do a necropsy. :/