Apple Watch 6 digit code not displaying

First, did your son follow all the steps in the "Before you sell, give away, or trade in your Apple Watch" section of:

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

For the six digit code problem, see the replies to:

No 6 digit code - Apple Community

No 6 digit verification code - Apple Community

Hopefully this will help solve your dilemma because I searched high and low for answers on this topic after trying everything else unsuccessfully. My Apple Watch series 1 got water into it and stopped working, so my brother gave me his unopened series 1 as a replacement. It wouldn't paid with my iPhone XR. Turns out it's bc the software/iOs on my iPhone is too new for the "old" original series 1 watch.

What I had to do was try to pair it with my old iPhone (the one I last used 2 years ago and was in a drawer in my basement in case I ever needed it!). While in the pairing process, it said it had to update the software on the watch, which is EXACTLY what I wanted it to do so my newer version iPhone would be compatable with it. Once the software on the watch was updated, I was able to pair it successfully with my current iPhone. Hope that helps!