Ball won't eat

[sherlock]

I've had this snake 12 years and he has always been an eager eater. The last 6 months he has only eaten once! He is active, always crawling around at night but when I put him in with a rat he simply looks the other direction, or tonight he crawled right over the rat! ( this is a live rat) I'm not overly worried, he acts healthy and has only lost a little girth. I would like to know if anyone's seen anything like this... Because in the 12 years I've owned him I've never seen this.