I want to disable a certain number, not to make regular and WhatsApp outgoing calls on my sons phone. Is there a way, I can do this , without him knowing about it?
EDIT: It is impossible to do this without your son knowing, as acejavelin said. This answer is the second best thing.
If I think what I think you are saying you, want to block out going calls on your phone? Here's a way to do it:
To block a contact that is saved in your phone
Open WhatsApp.Tap the Menu Button > Settings > Account > Privacy > Blocked contacts.Tap the add contact icon.Select the contact you wish to block from the list.
To block a number that is not saved in your phone
Open the chat with the unknown contact. Tap Block.
When you receive an initial message from an unknown number, you will also have the option to Report spam, which will report and block the user.
To block calls on your phone: It varies phone by phone, but here is instruction for some commons OS versions:
For Marshmallow or Nougat, open Dialer, go to your recent calls list, find the number you want to block and select Block/Report Spam. (If you don't want to report the number as spam, you can uncheck the box.) Then tap Block.
For Lollipop, go to the Phone app and select Call Settings > Call Rejection (ouch) > Auto Reject List. Type in the number or search for it, select it, and you're done.