I have an LG cu720 Shine and my son gave me a used bluetooth. The phone reconizes the bluetooth but now I need a code. How do I get that?

Most bluetooth headset use the pin code of 0000 and sometimes 1234

When you pair with another phone, you need to enter the last 4 digits of the other person's phone number
here are the steps
1.) Put your son's phone in discoverable mode

2.) Start Searching For BlueTooth Devices on your phone

3.) Click your sons phone

4.) Enter the last 4 digits of your son's phone number as a code

5.) Once your son's phone asks you for a passcode, enter the last 4 digits of your son's phone number once again

6.) Then they should pair