Hi,
You can create a form with the following details:
1) Asking the employee to tick on areas of concerns like/reasons for leaving
a) Better salary
b) Work environment
c) Relationship with peers/superiors
These are a few points; you can create an exhaustive list and ask the employee to tick on these. This will help you know what the areas of improvement are for the organization.
2) You can list the areas he/she enjoyed the most.
3) Things that disappointed the most.
4) Would he like to recommend somebody to this organization? (This is important as it will help you know what they think about the organization.)
5) Create one more list where you can ask employees to tick on areas which will encourage him to stay back like:
a) Better policies
b) Appraisal process
c) Empowerment, etc.
6) Is he interested in staying back or what will motivate him to stay back?
These are some points that will help you to create the exit form. Most importantly, take an exit interview before he is relieved. As HR professionals, we should try to speak to the person and retain him, not just ask him to fill the form, as while interacting, we get more information.