"I'm not comfortable with your demonstrated knowledge of procedures as FO and Pilot Monitoring. I'll take all the legs until you can show me you know how to do your job correctly."
Thus marking the point where CRM has been irretrievably lost for the remaining sectors.
I've found that a gentle enquiry along the lines of "are you feeling alright today?" or a "a bit under the weather?" can often stimulate greater enthusiasm; if I was being challenged in such a way, suggesting that my colleague had noticed something unusual behaviourally, I'd certainly pick up my game. If, on the other hand the response is belligerent or defensive, that may well reveal an underlying problem. You've got to accept as well that some of the youngsters today just don't seem interested in anything unless their face is welded to a phone or other electronic transmitting device.