Im trying to set my Apple Watch to my sons phone, but I can’t remember my user name and password

What did you sign into this forum with?.

If you forgot your Apple ID

Or;

You can ask apple to remove the activation lock online on this page;

Remove the Activation Lock Here.

Or this;

If you can't remember/gain access to the apple id that paired the watch to the phone.Take the watch and it's receipt with the serial number along with some photo id and any other proof of ownership to an Apple Store and request that they remove the Activation Lock for you.