"Call him and counsel him for his issues for looking for the job outside. Try to resolve the issue and bring confidence in him to retain the same."
If this was the first time, the employee has done this, I would take this option. However, I will also consider how long this employee has been working here, what his job performance has been, and generally how he gets along with coworkers, supervisors, etc. Definitely, he will be on the watch list from now on.