Best Gearing for a very fit but on his way out 52 year old
I use Sheldon Brown's Gear Calculator whenever I'm looking into gear ratios. For someone who doesn't ride any big hills I would think that 50/34 - 12/28 would be absolutely ideal. You could go 1x and get 98% of a 50/34 with a 44+10/42. See SRAMs pocket guide 

50-12 tops out at about 33mph definitely not something that mortals can sustain for very long on a flat road.