[HighlyOpinionated]

My son, for example, is a natural born citizen of the US and a citizen of at least two other countries. He will be Constitutionally qualified to be President at age 35.

Your son was born on US Soil to two parents who are legal immigrants? If Yes, then he is a Natural Born Citizen . . . but at age 18-21 he will have to declare his citizenship with the USofA and abandon citizenship to the other two Nations to be eligible to be POTUS.

As I read the Constitution.

[Citizen Blade]

Your son was born on US Soil to two parents who are legal immigrants?

Born to one natural-born citizen and one naturalized citizen.

but at age 18-21 he will have to declare his citizenship with the USofA and abandon citizenship to the other two Nations to be eligible to be POTUS.

That's not correct. There is no requirement under US law for him to do anything, ever, to retain his American citizenship or his natural-born citizenship status. At birth, at least two other countries considered him a citizen (and maybe 3, we're looking into it). We will need to fill out the appropriate paperwork with the embassies of those countries to make sure that they are aware of his birth and recognize him as a citizen, though that is just administrative.