I understand that they require 2/3 of the states to ratify an ammendment. That will not matter.
If he was forced to relinquish citizenship as a child to become an Indonesian citizen - and is therefore no longer a natural born citizen - they will pass a law that states natural born children are incapable of deciding for themselves and therefore cannot relinquish their status until the age of consent.
Basically they will state that a parent may relinquish citizenship of their child but the natural born status returns if the child once again becomes a citizen of the US.
The only thing they cannot overcome is if he was actually born in Kenya as a Kenyan citizen. No getting around that....but they will do their best to find a way.
There is zero chance he will not be sworn in on time in January. None. Whatever political manuvering or language contortions they have to do to get him there, he will be there taking the oath.