As a molecular biologist I would almost want to investigate if somehow the total expression of, or accessibility of this man's third Chromosome 21 is somehow reduced because this person seems to be very close to normal (2xchr21) humans on the normal to severe Down syndrome -spectrum. I have seen people with 2 times 21 with a more Down syndrome-like face than this man's even.

Mosaic trisomy, where only a subset of cells have extra chromosomes, is possible for essentially any chromosome including 21.

This makes some trisomies survivable, e.g., trisomy 16 which is otherwise incompatible with live birth.