Why? There is nothing illegal about an American holding multiple citizenships or passports.
But he is inelligible to be president.
My son is a natural-born American citizen. He is also, from birth, the citizen of at least two other countries (maybe 3). In addition to an American passport, we will get him at least one more passport when he gets older.
None of this will in any way prevent him from some day serving as President.
You’re simply uninformed.
Dual citizens cannot serve as president.