Handing down iPhone 12 to a son with an iPhone X.

I live on the east coast and just finished transferring my data from my iPhone 12 Pro to the new iPhone 14 Pro. My oldest son has the iPhone X and he lives on the west coast. I would like to send him my old 12 to replace his X. His is dying (or so he tells me).

What is the recommended process for me to make my old 12 ready for his transfer of data from his "expiring" X to the one I'm sending?

And are there directions that I can share with him that walk him through the process of exchanging his data from the X to the 12 easily. I'd bet my weight in Lobster tails he has never backed his phone up.

Thanks in advance for the advice.