How can I unlock my sons I phone if my ex wife set it up and refuses to tell me the password

Which password? If you are referring to the screen unlock passcode, you cannot. You can restore it, and set it up again to remove the passcode. click here ➜ If you forgot your iPhone passcode - Apple Support

You will need the Apple ID password however to complete the restore process.

If you are referring to the Apple ID password, you can try to reset it by following the steps here our requesting an Apple ID recovery. click here ➜ How to use account recovery when you can’t reset your Apple ID password - Apple Support