If your employee is returning to work, then you will use the P45 to add him on to your payroll software again.
You will need to process any payments made and then if he leaves the employment, you should follow normal procedures and generate a new P45.
If he is not returning to work, and you are simply paying final wages holiday pay etc, then you do not need to reinstate him.
Any payments made, will be treated as payments after leaving and are reported as such via your payroll software.
You can find guidance on payments after leaving here:
What to do when an employee leaves
Should you need any further assistance, please contact the Employer Helpline on 0300 200 3200.