How can I use a 2012 Macbook without any available password

As the original owner, it is your son—not you—who needs to handle this. He can start here:

If you forget your Apple ID or password - Apple Support

and

If you forget your login password on Mac - Apple Support

Think about how huge the black market would be if you could do what you want to do.

Your son needs to do this prior to giving the computer to you:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

If this is not properly completed by your son then the advice provided you by Apple is very good advice.

So, you think a thief who has stolen a Mac....new or old....but has no idea what the password might be....should be able to override the protection that was built into the Mac and reset the password or erase the Mac so they could use it as their own or sell it to another user?

If you could do this....so too could any thief.

If only they could make a car with those same protections where only the owner can drive it and has the power to remove the protections to allow someone else to use it. It really is not unbelievable that the car could be a perfectly functioning machine with software that could not be bypassed to prevent unauthorized access. We can only hope that day will come.

That would be your son that needs to gain access to his account if he does not remember the password.