I think that as a US citizen he is not allowed to become UN Secretary General, that position - in the infinite wisdom of the UN's founders - being reserved for citizens of nations not members of the UN Security Council.
So he can still do it, but he will have to re-activate his Indonesian or Kenyan citizenships, first. :)
“...he can still do it, but he will have to re-activate his Indonesian or Kenyan citizenships, first. :)”
Did he ever deactivate them?