The only way the FO can "Legally" log PIC time is if he already has a PIC type rating and is the sole manipulator of the controls. If that's the case, he has a legal right to log the time as PIC.
However, it has been discussed her extensively that not a single airline would accept it. They only accept time that was PIC when you were the captain.
Therefore, while you could legally log it, there would be no point since no other airline would accept it.
What the FAA allows you to log and what the airlines accept are two completely different things. I've sometimes thought I should keep 2 logbooks - one for FAA times and one for Airline times.