Add his user id as an authorizer in ‘Purchase authorization basic data’ screen, Authorizers tab. And then the authorizer should be added to the relevant Authorizing Routing template (template used for the relevant ‘Purchase order Authorization rule’ which is fetched when trying to release the PO) in ‘Authorizing Routing templates’ tab. 

Hi Nagah,

You can try the below steps.

1.Go to Person window and make sure his Person record is in state Active, and an user id is connected.

2.Go to Purchase Authorization Basic Data, Select the company.
3.Select Authorizers tab. Create new record, select above person id in Authorizer ID field.
4.Select Authorization Role as 'Authorizer' and save.

Next, you have to define Purchase Order Authorization Rule and connect above authorizer to that rule via Routing template id.

Hope this helps.