I'm no fiddler but I've had the same problem of trying to explain what I want from an instrument I don't play. Have him play long notes, with just a little of the bowing/ ornamentation that makes a fiddle sound like a fiddle. Harmony, fills, trills... as long as it doesn't tread on the vocals. Make him listen to recordings that feature fiddle backing up singers. Hope that helps!

Cheers,
Michael