Our first officers are selected and trained to be captains.
Why don't you make all your co-pilots captains then, and rotate final responsibility each leg? Lower your salary, raise his..
When he changes to the left seat, do you expect him to suddenly be able to perform max x-wind landings after years of not doing any?
No, I expect him to build up his own comfort zone gradually. I expect him to divert if he's not comfortable with the weather.