I don't know if it helps, but cats are usually very fickle creatures who whilst liking their home and owners would be happy to have a look at fresh fields now and then. My female cat used to go off for a week every summer for many years. One of my friends cats went missing and turned up over a year later 70 miles away. I suspect he fell into a van or something but he was unharmed and although a bit feral, returned to being the cuddly friendly cat that he always was.
It doesn't necessarily mean that something bad has happened to your pet- just something different. Its still fairly warm for autumn so he may be out on an extended mousing trip as theres plenty of wildlife out and about still.