Messaging from my Mac

I have two numbers associated with our Apple ID, one is mine and the other one is my son's. When I message on my Mac, the messages have his number associated with my messages. How do I change it so that the messages that go out and come in are from my phone number? I want him to have the ability to message on his phone, but I also don't want confusion (of friends and family) when I message from my MacBook Air on who the message is coming from.

Thank you for any help that you can provide!