No doubt he is a citizen of those places, which is not his doing (IF IF IF the citizenships were obtained when he was underage). So nothing to see there. UNLESS he did ANYTHING to promote citizenship in those countries after 18. Like apply for a passport, or even a passport renewal. It would still be legal but probably would not have made him a very attractive candidate for President of the US of A!
But dual citizenship is allowed. Any country can grant you citizenship.
Dual citizenship is fine for everyone EXCEPT those who want to be President. Natural born citizenship prohibits such split allegiences.