What age is the FO, and how long has he been in post as a FO?
A number of possible reasons could account for his MO............
From mental health, love life, family illness to name but a few
TO
Financial, passed over for promotion.........
You need to find out why, and soon. In the event of an incident/accident then it will all come out, and questions will be asked why a more proactive account was not taken to mitigate the issues..
Always easy to be wise after the event