Why does my MacBook autofill my son's contact details?

Re: My Macbook thinks I'm my son. What's happening how can I sort this out?

Wondering if you are signed in (to your Mac) with your correct Apple Account log-in and Apple Account password:

You may want to check: Sign in to your Apple Account - Apple Support

Thank you kindly for checking and confirming the sign-in details are correct.

Re: "My MacBook keeps changing my My Card to my Sons contact details. Auto fill brings up his details .

As you suggest: Seems that Contacts card is where the problem stems from.

Re: I keep having to go into contacts to change it but it doesn't stay changed".

Wondering why changing the Contacts card isn't working for you ?

There is a bit of a knack to the Contacts "edit" method on Mac.

( I've lost changes myself, as I often learn the hard way.)

ie: Before making changes, the "Edit" button must be pressed.

After making changes, they will not be saved unless "Done" button is pressed

More info: Update contact information in Contacts on Mac - Apple Support

If this hasn't helped:

It is not easy to help trouble-shoot this situation via these messages, but we may find a solution by patiently eliminating various possibilities.

Else: If you wish you could contact Apple Support (online chat / phone / and more):

You can click "Get Support - start now" at the bottom of the website below:

Mac - Official Apple Support