I've been keeping snakes for a long time & I have to say, I've never seen or heard of a snake eating their feces. But strange things happen in nature sometimes.
I think if I were in your shoes, the next time he defecates in his cage, I'd be tempted to leave it for just a bit & see if this happens again. I'd just have to know...

One other thing occurs to me that's also unlikely but would make more sense than him eating his feces: is it possible that he regurgitated, then re-consumed it?
A poorly-digested rodent can look like a log of poo, & I guess would still smell like rodent to him? Not a pretty thought either, do keep us posted?