Setting up my sons Apple ID on my old phone but my contacts still on the phone how do I remove

You should restore the phone to factory state before setting it up for him:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

See also the directions in this article:

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

See here for what to do before giving a phone away: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Note that an iPhone 6 cannot update past iOS 12, so it will not be able to run most current apps that require iOS 13 or newer.