You say he was a good worker up until a couple of weeks ago.
If that is the case then let a couple days go by and call him ask if everything is ok. Maybe there was a death in the family, or some other bad news in the family. Some families are tight and something may hit someone else harder than others.
Some people automatically think the worst about an individual...he's a piece of crap, he's worthless, workers like that should be slapped around blah blah blah blah blah blah and blah.....
I'm not saying be a counselor or a best friend but calling and asking isn't going to hurt and being good help is hard to find, it may help both of you.