My personal choice would be the Pixel 2.
Why? First, he's familiar with the Pixel line, so it will feel like just a better version of what he has. The updates (if he wants to download them instead of waiting for them) are in the same place, just a little higher up on the page. And it's a good phone. (If he does any kind of photography, everyone is comparing their cameras to the Pixel 3, and the 3's camera app has been ported to the 2. The 2 has the Visual Core, so almost-no-light photography is there, portrait mode is there, HDR+ is there - it's just a better OP [Original Pixel].)
And Google support makes someone who used to run customer service operations jealous - they're better than good. Lenovo? I have no idea, but most Chinese companies leave at least a little to be desired. (If the 4 offered anything interesting, I'd upgrade - that's how impressed I am with Google.)